Also clarify in GRXWriter.h that the writeable is released whenever the
writer finishes.

And move it to the RxLibrary.

Fixes https://github.com/grpc/grpc/issues/2194 .

- Moves them to the root of the repo.
- Makes them pass lint (except for the release tag warning).
- Includes the script to rename time.h and string.h in the spec.
- Fixes the #includes in the wrapper files.