[RFC] Add `feedkeys` API function part 2

Cleaning up after #916

- I've split the f_feedkeys() function from eval.c into a reusable feedkeys()
- Now the API vim_feedkeys should work just like the vimscript feedkeys(String keys, String mode)
- Added new vim_remote_send(String) API for compatibility w/ Vim --remote-send

> More semantics cstr_cast_string

Also good, though `cstr_as_string` has grown on me in the short time of its existence.