Move FrequencyInferer out of libresolution
Discussed briefly, FrequencyInferer doesn't benefit much from cython, isn't needed elsewhere in tslibs, and with this move loses the dependency on khash.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=h1) Report
> Merging [#21992](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2d0c96119391c85bd4f7ffbb847759ee3777162a?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `98.05%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21992/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21992      +/-   ##
==========================================
+ Coverage      92%   92.02%   +0.01%     
==========================================
  Files         170      170              
  Lines       50553    50705     +152     
==========================================
+ Hits        46510    46659     +149     
- Misses       4043     4046       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.42% <98.05%> (+0.02%)` | :arrow_up: |
| #single | `42.31% <74.67%> (+0.09%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/21992/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `97.08% <98.05%> (+1.17%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=footer). Last update [2d0c961...ebb6d52](https://codecov.io/gh/pandas-dev/pandas/pull/21992?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
