Get past.tests.test_builtins running on Py2
Improvements to past.builtins: map, filter, etc.

These now pass more of the Py2.7 unit tests
Get all tests in past.builtins.test_builtins passing again on Py2

This disables the execfile() reimplementation on Py2. The Py3 one is still
broken.
Fix issue #38 (newint division)