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)
function result is meant to be transient, and therefore should not be regularly storeable by the user... but there is no way to make a "and you can't have anything but a temporary to this" type, as far as I can tell.
sol::object had a few reference leaks in the way it retrieved values: it now does it properly without leaving the stack at +1 item
sol::stack was drastically cleaned up, with the following key change:
* sol::stack::push now returns an integer of the number of things its pushed (usually 1, but can be more) (Thanks, @PrincessNyanara!)
* sol::stack::call now calls functions flexibly, and getting is done more reliably
* due to the innovation of stack::call and using absolute indices, we no longer have to use reverse_call style programming to deal with lua
* sol::reference::get_type is now const-correct
* sol::state and sol::table now have a cleaned up `get` implementation since it is no longer held back by the ugliness of VC++'s incapability to handle templates
* the name `sol::userdata` now belongs to a type that actually encapsualtes a void* with a pusher/getter than gets a userdata void* value (TODO: give it a template to make it static_cast to that type on get?)
* lightuserdata_t -> light_userdata, upvalue_t -> upvalue as type names (mostly details)
* pushers for various types were updated to return integers
Userdata now properly forwards arguments to constructor
get_call now properly has extra parameters to allow for forwarding items from the first call that are not popped
Added tests to cover new cases
Fix for userdata to accept base classes where the derived class uses the name of a base member function to access it (Derived::get_num, where get_num is only implemented in Base::get_num)
VC++ makes this acceptance easy by taking the name as "Derived", but GCC and Clang
