Char table of functions to search for the word boundary.
Each function is called with two arguments; POS and LIMIT.
POS and LIMIT are character positions in the current buffer.
If POS is less than LIMIT, POS is at the first character of a word,
and the return value of a function is a position after the last
character of that word.
If POS is not less than LIMIT, POS is at the last character of a word,
and the return value of a function is a position at the first
character of that word.
In both cases, LIMIT bounds the search.