The magic variables '$~' and '$?' are now method-local and concurrent-safe. 'Tuple' is now correctly considered a struct. 'Pointer' is now correctly considered a struct. Renamed 'Function' to 'Proc'.

Fill argument types when passing a function pointer to a C function (like in `->foo` or `->Foo.bar`). Also, renamed `MacroType` to `TypeNode` because now its also used outside macros.

Fixed a parser bug where a hash literal of namespaced values couldn't be created ({Foo::A => 1, Foo::B => 2})

Fixes #384.

When issuing a "can't send closure to C function" show the closured variables if the compiler knows them

Make C structs and unions inherits from Struct. Make it work so you can invoke `to_s` on them (for unions it just prints the name of the type).

I believe it is important to keep the names consistent everywhere, even
the tests. If our direction moving forward is to use the LibX
convention, then that should be reflected everywhere.

Allow T::Bar when T is a generic type var. Also give an error when T is a union type (instead of crashing the compiler :-P)

Lexer objects are based on the concept of page objects for integration
tests. They abstract the intricacies of testing the lexer, and provide a
DSL like syntax for the actual tests.

This serves the purposes of reducing boilerplate in tests, improving
tests readability, removing duplicate knowledge throughout the tests and
thus following SRP.

I extracted the lexer object into it's own class and file, because I
feel it will have different reasons to change than the actual tests.