Allowing shallow_copy/deep_copy to accept any type makes it easier to
write generic code where you just want to copy some input without
knowing anything about it. Like an event system, save system, etc.
Add corresponding test.
Tests pass
Replace copy with shallow_copy, deep_copy.
Replace overlay with shallow_overlay, deep_overlay.
Significant rewrite of these functions and their semantics to update to
the note above them.
Add corresponding tests.
The second loop doesn't care if the values are equal. It only checks if
the keys are there. The first loop already checked for equality.
Add tests to prove correctness using testy.lua.
