Implement Mixins

[mfichman]

Implement mixins, for interfaces and regular types. Mixins will basically just "paste" the mixin code into the body of the class. Debatable: Should mixins inherit from "Mixin?" I think yes, to make the compiler simpler and to distinguish between usable objects and Mixins, which are not standalone.

Conflict rules: victory goes to the method defined in the Object, or to the Mixin listed first in the mixin list.

Self: self-types should be usable in Mixins, and they should be bound to the type of the Object they are mixed into.

Implementation: just copy, or perform some kind of lookup? Should multiple functions be generated, or should the vtable forward to the mixin implementation? (need to worry about instance variables -- should they even be allowed in Mixins?)

[mfichman]

Instance variables in Mixins can only be accessed by virtual function calls. This will simplify the implementation, since no data inheritance is required (only code inheritance, which is the important part anyway).

[mfichman]

Mixins are implemented, except for the following rules:

• All objects must mix-in one and only one of: Mixin, Object, Interface, Value
• Mixins cannot directly access attributes
• Values cannot have mix-ins (because they have no vtable)
• Interfaces cannot have mix-ins (because they have function implementations)
• Objects/values can't mix-in interfaces
• Interfaces can mix-in other interfaces
• Mix-ins can't have constructors

Possibility: Combine Interfaces and Mixins?

Also, code-gen for mixin functions must treat "this" as an interface.