Reenable add_to_input_buf* functions

- Remove guards around add_to_input_buf and add_to_input_buf_csi
- Remove hangul related code
Return size from add_to_input_buf

- add_to_input_buf() and add_to_input_buf_csi() now return the
  total amount of bytes written
Enable vim_push_keys API function

- This commit implements the vim_push_keys() API function by append
  the given string to the input buffer - using add_to_input_buf_csi
Clean up a bit

- Add return comment
- Change len types to size_t
- Fix broken return value