Changes by AMK:

Removed handling of \e, \cX escapes, following a string-SIG discussion.

Fixed minor typos in re.py
re.error is now set equal to reop.error.
Move definition of constants like NORMAL and CHARCLASS into reop, which
  exports them; re.py was changed to import them from reop.
Added C equivalents of _expand and expand_escape to reop, and changed
  re.py to use them.