Added abstraction for finding first mismatch between two string.

This abstraction will be used in the string comparison functions.  We
are trying to optimize for long strings, so the comparison will be
dominated by the time it takes to find the first position with
different characters.

This version compares two simple strings using char=.
Find mismatch between simple and general string using char=.
Find mismatch between general and simple string using char=.
Find mismatch between two general string using char=.
New mismatch functions using char-equal.