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
This should enable operator[] to work in all cases that it is to be expected, thanks to a lot of explicit conversions and some basic overload resolution ranking tricks (e.g., making some conversions a template).
We also need to remember that GCC expects `. template (function name)` in generic code, as it can parse things in a really dumb and silly manner.
