WIP: Rework IndexEngine to better support hashtable-free algorithms.
Initial work toward #14273.  This makes `__contains__` behave uniformly across all index types by making it simply defer to `get_loc` (we can and probably should go back and put in a fast path for the case that a hash table has already been populated).  Assuming this general path seems reasonable, the next step would be to update `get_indexer` to do binary-search-based lookups on large indices.

A few high-level design questions:
- I haven't benchmarked yet to see how big the difference is, but theoretically these changes are trading slower lookups for a smaller memory footprint.  Depending on users' usage patterns and resource constraints, the tradeoffs made here may or may not be an improvement for them.  I added an `avoid_hashtable` keyword to IndexEngine to make it easier to test the new behavior interactively without having to work with giant indices.  Do we want to support a user-configurable toggle like this for cases where people know that they really do or don't want a hash table?  If so, where would they pass that option?  Alternatively, should _SIZE_CUTOFF be configurable, perhaps via `pandas.set_option`?
- One odd behavior I ran into while working on this is that `Int64Index.__contains__` silently truncates floats to integers before doing lookups, while `Int64Index.get_loc` does not.  On this branch, that behavior is only preserved for integral-valued floats (the behavior of coercing integral floats to ints was explicitly tested).  This also happens for `DatetimeIndex`.  Is there any concern about "breaking" the behavior of `Int64Index.__contains__` for non-integral floats?

<details>

```
In [11]: pd.__version__
Out[11]: u'0.19.0'

In [12]: i
Out[12]: Int64Index([1, 2, 3, 5], dtype='int64')

In [13]: 1.5 in i
Out[13]: True

In [14]: i.get_loc(1.5)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-14-c37f93fbd72a> in <module>()
----> 1 i.get_loc(1.5)

/home/ssanderson/.virtualenvs/pandas-19/local/lib/python2.7/site-packages/pandas/indexes/base.pyc in get_loc(self, key, method, tolerance)
   2104                 return self._engine.get_loc(key)
   2105             except KeyError:
-> 2106                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   2107
   2108         indexer = self.get_indexer([key], method=method, tolerance=tolerance)

pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4160)()

pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:3983)()

pandas/index.pyx in pandas.index.Int64Engine._check_type (pandas/index.c:7773)()

KeyError: 1.5
```

</details>
- [ ] closes #14273
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Some fairly simple/naive benchmarking suggests performance actually improves if the index is decent sized.  So assuming this example is representative, you probably could always do binary search above a cutoff, not needing to expose that option.

``` python
In [1]: idx = pd.Index(np.arange(0, 100000000, step=2))

In [2]: idx.get_loc(250000)
Out[2]: 125000

In [3]: %timeit idx.get_loc(250000)
The slowest run took 4.52 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 8.64 µs per loop

In [4]: %timeit np.searchsorted(idx.values, 250000)
The slowest run took 34.45 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 1.38 µs per loop

In [5]: %timeit idx.get_indexer(np.arange(0, 100000000, step=10))
1 loop, best of 3: 4.43 s per loop

In [6]: %timeit np.searchsorted(idx.values, np.arange(0, 100000000, step=10))
1 loop, best of 3: 416 ms per loop
```
 I think we need a set of tests to validate whether the hash tables are instantiated. e.g. something simple like this can work.

```
    def get_state(self):
        """ return my internal state """
        return {attr: getattr(self, attr) for attr in ['initialized', 'monotonic_check',
                                                       'unique', 'unique_check']}
```

Then you can have a set of tests which assert the internal state of the IndexEngine (as you do operations).
 @ssanderson haven't looked at this in a while.  

can you rebase / update? @jreback I'm still interested in this, just haven't been able to dedicate a solid block of time to it.  I'll try to at least rebase against master this week. note that the index.pyx was mostly moved to templates (so this is in fact more generic now) closing for now @ssanderson but feel free to update if you can