[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 the best semanthics but I could add
```C
String cstr_as_string(char *str)
{
  return (String) {.data = str, .size = strlen(str)};
}
```