And move it to the RxLibrary.

With GRXWriter a subclass of all writers, we can map the requestsWriter directly
without a dumb wrapper, and return GRXImmediateWriter objects where GRXWriter is
expected.

Part of fixing https://github.com/grpc/grpc/issues/2192

So they aren’t mutated before being written later.

The stub is manually adapted:

- to the evolved runtime,
- as if generated with insertion points.

- 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.