BUG: Categorical.fillna with iterables
Closes https://github.com/pandas-dev/pandas/issues/19788
Closes https://github.com/pandas-dev/pandas/issues/21097

Note that `Series.fillna` still doesn't allow iterables. I'm not sure that we should allow that, as it's potentially confusing what that would do.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21215?src=pr&el=h1) Report
> Merging [#21215](https://codecov.io/gh/pandas-dev/pandas/pull/21215?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6abb6148c8dc14e6279f31f2e620ecf52822107?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21215/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21215?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21215      +/-   ##
==========================================
+ Coverage   91.84%   91.84%   +<.01%     
==========================================
  Files         153      153              
  Lines       49505    49506       +1     
==========================================
+ Hits        45466    45467       +1     
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21215?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21215/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.68% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21215?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/21215?src=pr&el=footer). Last update [f6abb61...43c185e](https://codecov.io/gh/pandas-dev/pandas/pull/21215?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 