`set_function`/`set_usertype` now properly use `set`
For the time being, we are going to avoid implementing `traverse`; aside from performance of nested table access (e.g. `int x = lua["a"]["b"]["c"]`), it's a pain. ;~;
additional tests to make sure pass-by-value and copy semantics work as intended
new proxy_base class to reduce code duplication
update function / protected_function usage (to solve starwing's issue while keeping code as clean as possible)
