Overloaded functions now work properly when types are specified in signature
this triggered overhaul of set_function/pusher<function_t>::push(...)
both state and table reflect changes to userdata structure to make it easier to use
tests updated to account for overload resolution
some function-related traits added to make use easier -- cleaned up archaic typenames in function_types.hpp
Account for std::reference_wrapper for objects -- sol now uses copy-by-default (value-semantics) for all functors
updated tests to reflect this