let bindings

[VonTum]

Since we're now on fully Hindley-Milner based typing, we can actually create let-bindings.

This should be a rather easy fix.

The fun thing is, that it doesn't conflict with name-resolution-dependent type/submodule resolution. With a let binding, of course, we have to assume it'll always be a type, and not a submodule.

Actually, it's not possible to not name a submodule in the code. Because we're accessing fields and interfaces, but those names can't tell us which submodule it is.