Add tests for implements/interfaces
Add tests for implements/interfaces

These tests just check some basic assumptions about the way interfaces
are implemented using the `implements` primitive

- They can be implemented with functions of any name.
- They can be implemented retroactively.
- `Implements` declarations can precede definitions.

eriksvedang/Carp,carp-lang/Carp,eriksvedang/Carp,carp-lang/Carp,carp-lang/Carp