merge count_size_as_multibyte, parse_str_to_multibyte

merge count_size_as_multibyte, parse_str_to_multibyte

* character.c, character.h (count_size_as_multibyte):
Renamed from parse_str_to_multibyte; all uses changed.
Check for integer overflow.
* insdel.c, lisp.h (count_size_as_multibyte): Remove,
since it's now a duplicate of the other.  This is more of
a character than a buffer op, so better that it's in character.c.
* fns.c, print.c: Adjust to above changes.