Add some arg passing tests. (Not final).
First attempt at new argument passing semantics.
Py->PHP: Test passing objects by reference and value.
Simplify some tests.
Py->PHP: test strings by value and reference.
Test a couple more arg passing cases from php to python.
Improve comments regarding why we must pass a reference when calling python.
If you pass to PHP a value argument by reference, fail.
Ensure that passing a value to a pass by ref arg is an error.
Don't copy Python list when passing to PHP by reference.
Duplicated import
Implement copy() on wrapped Python lists.

This allows PHP to do it's copy-on-write dance and means that we
don't have to clone when passing by value. See pypy fork for a
corresponding commit.
Similarly, Implement copy() on wrapped Python dicts.

Allows copy on write in PHP.
Test dict codepaths more thoroughly.
Remove unused copy kwarg from to_php().