Added pop() function to `reference`
`stack.hpp` now has many more functions to properly handle user data
`types.hpp` now has `userdata_t` and `lightuserdata_t` to faciliate `stack`'s `pop` and `push` operations
Class binding functionality built into `table` and `state`, as well as placed in `userdata` class in `userdata.hpp`
demangling detail now present for clang, gcc, and MSVC (somewhat for MSVC)
Constructor arity still needs to be handled for `userdata<T>`
tests updated and passing
