Enforce that abstract members are implemented

[bioball]

This adds a check that abstract members must be implemented.
If any members lack an implementation, an error is thrown describing
the missing members.

Some open questions here:

1. Should we get rid of abstract properties? Semantically, they're kind of strange because class properties already don't need an implementation.
2. Should we require the override modifier when overriding?
3. What should we do in the case of overriding properties that don't add a type annotation? Currently, we say "these properties don't contribute to the class definition", which means foo = 1 should not be a valid implementation of abstract foo: Int.

Closes #1262