a lua upvalue can be lightuserdata, userdata, or anything else that can have its address taken (it's immediately popped of the stack and carted around with function call)
a lightuserdata can only be a pointer (void*)
a regular userdata can be anything, but is stored as void* because of "anything" semantics and C heritage of lua
upvalues deserve to use the `lua_upvalueindex(n)` macro: lightuserdata/userdata does not (must not) go through this process
get turned into getter<T>, matches pusher<T> and uses same semantics as std::allocator and other things used throughout the codebase
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userdata has its traits defined outside in new file of userdata to prevent errors when trying to use those typetraits in places before userdata.hpp gets included
userdata was changed to support returning itself via pointers or references.
rework of stack changes semantics based on T&, T*, and T&& (the last one tries to create a new userdata and move in data)
solves problems maybe presented in https://github.com/Rapptz/sol/issues/25
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container.hpp is attempt at solving original problem before going on wild tangent with userdata, stack, and get
is going to attempt to use userdata to allow transporation of containers losslessly, perhaps without copying need
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found out trying to return a std::function does not work -- not sure what do exactly?
perhaps should push c closure as last thing, but right now it is tied to a key value (code comes from table.hpp and set_function)
will just have to think over how stack arranges itself and learn what to do
Refactoring on function_types.hpp performed to slim down some of the calls: could use more refactoring
Drastically simplified userdata's binding capabilities: constructor supports both `:` and `.` syntax (but member functions DO NOT).
All tests are passing
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
