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)
checkargs is now properly propogated through the "call" functions
tests now define SOL_CHECK_ARGUMENTS to make sure
the tests will always check arguments now as well (caught one minor implementation detail missing from that!)
