Add helper cstr_as_string()

- Add nocopy helper alternative to cstr_to_string
Refactor vim_feedkeys and f_feedkeys

- To clean up the mix between feedkeys and replace_termcodes
  the vim_feedkeys API function now does the same thing as the
  vimscript feedkeys() function
- The original f_feedkeys() function now calls the vim_feedkeys()
  function from the API
Add API vim_replace_termcodes

- Add API function to call replace_termcodes
Merge pull request #920 'Add `feedkeys` API function part 2'