Fix performance issues in replace_text(), replace_text_regexp(), and

Fix performance issues in replace_text(), replace_text_regexp(), and
text_to_array(): they all had O(N^2) behavior on long input strings in
multibyte encodings, because of repeated rescanning of the input text to
identify substrings whose positions/lengths were computed in characters
instead of bytes.  Fix by tracking the current source position as a char
pointer as well as a character-count.  Also avoid some unnecessary palloc
operations.  text_to_array() also leaked memory intracall due to failure
to pfree temporary strings.  Per gripe from Tatsuo Ishii.