Pitch: Introduce `for borrow` and `for inout` to provide non-copying collection iteration

[jayton (Jens Ayton)]

@GreatApe

OK but still, in other contexts inout is never attached to the variable, and not even present at the call site. It's an annotation used in function declarations, on the type.

This pitch is explicitly built on top of the one about local borrow and inout bindings.