REF: Move most remaining arith helpers to datetimelike mixins
Before we can finally move over `_add_datetimelike_methods` and be done with this, some more work needs to go into the `__new__` methods.  This gets another big chunk through, starts implementing tests.
looks ok to me. merge on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21815?src=pr&el=h1) Report
> Merging [#21815](https://codecov.io/gh/pandas-dev/pandas/pull/21815?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c697ca6023136bfb431c1674ba70b1f52a15d604?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `94.64%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21815/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21815?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21815      +/-   ##
==========================================
+ Coverage    91.9%   91.92%   +0.02%     
==========================================
  Files         160      160              
  Lines       49892    49914      +22     
==========================================
+ Hits        45852    45883      +31     
+ Misses       4040     4031       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <94.64%> (+0.02%)` | :arrow_up: |
| #single | `42.1% <36.6%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21815?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.08% <ø> (+0.02%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `94.24% <ø> (-0.16%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.1% <100%> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.17% <100%> (+0.16%)` | :arrow_up: |
| [pandas/core/arrays/timedelta.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YS5weQ==) | `92.47% <92.85%> (+4.81%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `95.83% <94.44%> (+2.2%)` | :arrow_up: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `93.68% <94.59%> (-0.04%)` | :arrow_down: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `87.83% <95.65%> (+2.11%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21815?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/21815?src=pr&el=footer). Last update [c697ca6...8b93586](https://codecov.io/gh/pandas-dev/pandas/pull/21815?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 