* str_utf_start/end both cast the offset into the utf string
to a char_u, a pointer + long is well-defined and the cast is
unnecessary. This previously resulted in issues for offsets greater than
256.
* str_utfindex checks number of arguments only, but ignores the case in
which the second argument is an explicit nil. Previously this required
dropping the second argument entirely.
* Modify the C binding to explicitly check if the second argument is nil
vim.str_utf_{start,end} return the offset from the current position to
the start and end of the current utf-character (nearest codepoint)
respectively.
