Inheritance needles

Implements Base and Derived needles (but under the names child and parent... )

Depends on #295
I don't care about the type nearly as much as what the function is supposed to do. For example, lambda x: x would meet this requirement but probably wouldn't do what you want. I bring this up to make a more general point: type signatures are nice, but they don't suffice—at least not without *much* more descriptive types.