Move custom rf class outside the combsimp definition

This fixes, or rather, works around, a bizarre caching issue involving classes
defined inside functions.  See the discussion on pull request #1710.
change rf to _rf in simplify
Merge pull request #58 from smichr/rf

change rf to _rf in simplify
Added regression test for issue 7425.

Make sure lambdify can handle Python reserved keywords as symbol names.
Merge pull request #7427 from moorepants/issue-7425

Added regression test for issue 7425.
Merge pull request #1959 from asmeurer/rf

Move custom rf class outside the combsimp definition