`#` == __len
`[key] = value` == __newindex
`key` == __index
are not working properly for some reason, will need to investigate more deeply to find out why
on bright side all tests pass including new tests added (for take/return std::function)
fixed some things forgot to change with addition of upvalue_t for clarity (see previous commit about userdata/lightuserdata vs upvalue)
get turned into getter<T>, matches pusher<T> and uses same semantics as std::allocator and other things used throughout the codebase
-----
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
-----
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
-----
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
