Add helper cstr_as_string()

- Add nocopy helper alternative to cstr_to_string
Split f_feedkeys into a separate feedkeys function

- This commit separates the feedkeys logic from the vimscript
  checks in preparation for using feedkeys() in C code.
Make vim_feedkeys a simple wrapper around feedkeys

- To clean up the mix between feedkeys and replace_termcodes
  the vim_feedkeys API functions now does the same thing as the
  vimscript feedkeys() function
Add API vim_replace_termcodes

- Add API function to call replace_termcodes
Use API function from f_feedkeys