Implement a simple type checker

[osa1]

This implements type checking in the interpreter.

The type system is simple:

• All type variables have kind *.

• All function declarations have full type signatures.

• No elaboration is necessary: method calls are implemented via (type, method) lookups.

• Traits have only one type parameter.

We also don't implement row polymorphism for anonymous types in the interpreter.

The result is a very simple type checker that checks each function in isolation.

[osa1]

Almost there.. we need to implement default trait methods so that __gt etc. will work when you only implement cmp.

[osa1]

I think we just need to check that the predicates hold after checking functions and that should be it! (for now)