Remove unnecessary iNaT checks from _Period properties
In the status quo, `pd.Period.year` goes through several layers of redirection, one of which checks for `self.ordinal == iNaT`.

```
cdef class _Period(object):
   [...]
    cdef _field(self, alias):
        base, mult = frequencies.get_freq_code(self.freq)
        return get_period_field(alias, self.ordinal, base)

    property year:
        def __get__(self):
            return self._field(0)

[...]
def get_period_field(int code, int64_t value, int freq):
    cdef accessor f = _get_accessor_func(code)
    if f is NULL:
        raise ValueError('Unrecognized period code: %d' % code)
    if value == iNaT:
        return np.nan
    return f(value, freq)
```

But `Period.ordinal` will never be `iNaT`, because `Period.__new__` returns `NaT` in that case instead of a `Period` object.  So we can skip the `value == iNaT` check in `get_period_field`.  With that out of the way, we can skip `_get_accessor_func` and `_field` and and just write in `pyear(ordinal, freq)`.

This PR changes this property lookup for `year`, `month`, `day`, ...

Speedups ~10% across the board.  Before:

```
In [2]: per = pd.Timestamp.now().to_period('M')
In [4]: %timeit per.month
The slowest run took 12.73 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.4 µs per loop

In [5]: %timeit per.second
The slowest run took 15.01 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.4 µs per loop

In [6]: %timeit per.year
The slowest run took 14.73 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 1.36 µs per loop

In [7]: %timeit per.is_leap_year
The slowest run took 16.94 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.37 µs per loop



In [8]: per = pd.Timestamp.now().to_period('ms')

In [9]: %timeit per.is_leap_year
The slowest run took 16.74 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.5 µs per loop

In [10]: %timeit per.minute
The slowest run took 13.71 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.37 µs per loop
```

After:
```
In [2]: per = pd.Timestamp.now().to_period('M')

In [3]: %timeit per.month
The slowest run took 19.96 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.21 µs per loop

In [4]: %timeit per.second
The slowest run took 15.26 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.25 µs per loop

In [5]: %timeit per.year
The slowest run took 16.55 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.21 µs per loop

In [6]: %timeit per.is_leap_year
The slowest run took 15.96 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.24 µs per loop

In [7]: per = pd.Timestamp.now().to_period('ms')

In [8]: %timeit per.is_leap_year
The slowest run took 15.72 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.33 µs per loop

In [9]: %timeit per.minute
The slowest run took 17.03 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.23 µs per loop

```



- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=h1) Report
> Merging [#17421](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1981b679b0619de0765c2009684ce4abd886189d?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17421/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17421      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49581    49581              
==========================================
- Hits        45198    45189       -9     
- Misses       4383     4392       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=footer). Last update [1981b67...df81546](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=h1) Report
> Merging [#17421](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0097cb712a7361a69eb4f5ebb9bc13c2b8733f19?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17421/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17421      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.02%     
==========================================
  Files         163      163              
  Lines       49586    49586              
==========================================
- Hits        45233    45225       -8     
- Misses       4353     4361       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (ø)` | :arrow_up: |
| #single | `40.2% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.53% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=footer). Last update [0097cb7...8e09c56](https://codecov.io/gh/pandas-dev/pandas/pull/17421?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can we remove `get_period_field` completely? > can we remove get_period_field completely?

AFAICT the only remaining usages are in tests.  It is exposed so technically _could_ be used by someone somewhere.  I'm trying to err on the side of doing-too-little in PRs to keep the reviewer(s) happy.
 can you add asv's for these Period properites (and then run the comparions). See additions to the asv period file.

```
asv continuous -f 1.1 -E virtualenv master HEAD -b period.Properties
[...]
BENCHMARKS NOT SIGNIFICANTLY CHANGED.
```

For once I actually believe the results! this looks like it overlaps with #17422 somewhat. ok with merging either first; but I suspect you will want to rework this one to directly call the cython routine (get_freq_codes) . lmk. > but I suspect you will want to rework this one to directly call the cython routine (get_freq_codes)

My preference would be to merge them separately-- they should not conflict-- and then in a follow-up to #17422 I'll make a pass through all of `_libs` to replace any calls to `frequencies.get_freq_codes` with the cython version. ok that's fine, let's fixup / merge #17422 first The Travis error is a greyed-out circle.  Not sure what to make of it.  Does this require action on my part? travis was hicupping. why don't you rebase and push again. thanks!