Rewrite the Ruby compiler's type system

Fixes https://gitlab.com/yorickpeterse/inko/issues/98

TODO

• Start by defining the individual type structures and write tests, instead of immediately hooking these up into the compiler.
• Remove memoizing from the type database as this won't translate well to Inko (it's also not really necessary).
• Get rid of Type::Boolean (somehow) in favour of just using Type::Object
• Remove Type::Nil in favour of using Type::Object and the std::marker::Optional trait. This requires that types have access to the type database so they can look up types in other modules.
• Split Type::TypeParameterTable into two separate structures: one for storing type parameters and one for mapping these to their instances
• Spec out how type compatibility with regular objects is actually supposed to work
• Return Type::Error instead of Type::Dynamic when producing errors
• Don't produce errors when sending messages to Type::Error to prevent cascading type errors
• Determine how we should initialise generic types
• Stop using the horrible Inspect module
• Change type parameter syntax from !(X, Y, Z) to [X, Y, Z]. This is easier to type (on a QWERTY keyboard it doesn't require the use of shift).