Drop Type Projections

[odersky]

We need to clarify exactly what abstract types are illegal.

[odersky]

In fact, the spec says exactly what is implemented: T # A is an error is T is an abstract type, no matter what A is.

[odersky]

See #5375 for a test case.

[odersky]

The precise semantics is: In T # A, T must be realizable. Actually, this would be a good place to add a spec of realizability, which is currently missing.

[ckipp01]

Hey @odersky is this still something needed or something you plan on doing? If not, is this something that can be closed? I also removed the SIP from the title just to avoid any confusion.