[WIP] Type classes

[peq]

This branch contains an implementation of type classes as described in the new generics design in #679.

TODO list:

• [x] Instance Declarations
• [x] Basic Type Bounds
• [x] Type bounds in classes
• [x] Dependent type classes
• [x] Efficient search + namespacing (only search for instances in imported packages)
• [x] implicit instances for classes: ToIndex, FromIndex (maybe: TableKey, TableValue, Hashable)
• [x] Type Check: All functions implemented correctly
• [x] Add check for diverging instance search
• [x] Also search for supertypes
• [x] Also search for methods in extended interfaces
• [x] Overloads with extension methods / type classes should not give error.

Maybe add:

• [ ] Complex inference (see Iterable example)
• [ ] Translate casting to int using ToIndex and FromIndex type classes (better backwards compatibility)
• [ ] Use type classes for iterators (for-loops)
• [ ] Use type classes for operator overloading
• [ ] Add an expression (or magic function) for obtaining type class instances and calling methods on them, like (implicitly<Show>.toString(42))
• [ ] Automatically use existing methods and extension functions in instance declarations

[Frotty]

Nice 👍 I'm not too sure about the name instance since we frequently use it in the documentation to refer to a class instance.

[peq]

Yeah, will probably change it as it also introduces a new keyword and thus breaks StdLib tests.

Maybe just implements Show<int> or for <T: Show> implements Show<List<T>>?

[Frotty]

I get this error when trying to compile forest defense: