[cl-unification-devel] Updated patch to cl-ppcre integration

This replaces my previous patch to cl-ppcre.lisp. In addition to the previous functionality changes/bug fixes, it converts 'unify to use #'cl-ppcre:scan-to-strings, which simplifies the logic somewhat. It also fixes a bug involving unbound register groups. That is: (match (#T(regexp "(foo)(bar)?baz" (?x ?y)) "foobaz") (values x y)) will now return "foo", nil rather than signaling a type-error.