[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

Not sure - what if a termcode evals to an empty string or Ctrl-V? 

**Nevermind** mind that would not be an error anyway. I'll remove the error from vim_replace_termcodes.