[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

@oni-link is right, it seems. Correct me if I'm wrong, but doesn't this also mean that the current code (as it stands) contains a leak? (`result` is never freed).

@mikaelj yes, in fact I wanted to add it when I made `cstr_to_string`. I wonder what to call it though, to set it apart from `cstr_to_string` but illustrate that it takes ownership of the memory. Perhaps call the original `cstr_to_string_dup` and the new one `cstr_to_string`?