Extract Unit to top-level

Extract Unit to top-level

Summary:
[Folly] Extract `Unit` to top-level.

It was in `folly/futures/`, but this diff moves it to `folly/`. It is needed for futures, but it is more general than futures and can be used separately.

Users must replace `folly/futures/Unit.h` with `folly/Unit.h`.

Also codemods existing usage sites:

```
hg grep -lw folly/futures/Unit.h | xargs perl -pi -e 's,\bfolly/futures/Unit\.h\b,folly/Unit.h,g'
```

Reviewed By: igorsugak

Differential Revision: D3314280

fbshipit-source-id: 16279b76b1d24529bec49077ccb36cd7d39f23b8