melt moved into its own module
This pull request is to make PR #17677 easier and was requested by @jreback [here](https://github.com/pandas-dev/pandas/pull/17677#pullrequestreview-73974610)

rebase

i would also move lreshape and wide_to_long to melt.py # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=h1) Report
> Merging [#18148](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d8129b49b76362ed148938bcb64e095a95aaf19a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `96.29%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18148/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18148      +/-   ##
==========================================
- Coverage   91.42%   91.41%   -0.02%     
==========================================
  Files         163      164       +1     
  Lines       50068    50076       +8     
==========================================
- Hits        45777    45776       -1     
- Misses       4291     4300       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.22% <96.29%> (ø)` | :arrow_up: |
| #single | `40.36% <15.74%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `100% <ø> (+0.72%)` | :arrow_up: |
| [pandas/core/reshape/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9hcGkucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <100%> (-0.1%)` | :arrow_down: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `96.19% <96.19%> (ø)` | |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18148?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/18148?src=pr&el=footer). Last update [d8129b4...f5860a1](https://codecov.io/gh/pandas-dev/pandas/pull/18148?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I would keep the code changes / clean-up for a follow-up PR, and keep it here to just moving things thanks @tdpetrou 

ok can you do a followup-PR to implement the small cleans before we move on to your proposed changes. thanks.