Allow returning function objects in object mode

Allow returning function objects in object mode. If function object is returned in nopython mode, an exception is thrown. Fixes regression in oldnumba/tests/builtins/test_builtin_abs.py
Huh...did we just add it? I swear that pow wasn't working in nopython mode a few days ago, but now it works for me. I'll change the test to use something else.