Fix the .find() method for memory maps.
Fix the .find() method for memory maps.

1) it didn't obey the "start" parameter (and when it does, we must validate
   the value)
2) the return value needs to be an absolute index, rather than relative to
   some arbitrary point in the file

(checking CVS, it appears this method never worked; these changes bring it
 into line with typical .find() behavior)

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator