Add a way to calculate modify operations

Some applications replace existing (large) values with new ones using WT_CURSOR::update, where there are many bytes in common between the old and new value. WiredTiger has the WT_CURSOR::modify interface to make such modifications more efficient, so it would be helpful to have a way to calculate a set of modifications given the old and new value.

This calculation should be fast and run in linear time, and favor speed over accuracy. It can assume that a large proportion of the old and new values match (at different byte offsets), and give up if a set of modifications cannot be found efficiently.