Either forbid or support implied bounds from type definitions

[Manishearth]

struct Foo<'a, 'b: 'a>(&'a u8, &'b u8);


fn foo<'a, 'b>(x: Foo<'a, 'b>, a: &'a u8, b: &'b u8) -> &'a u8 { b }

This compiles, since foo has an implicit 'b: 'a bound inherited from the struct. We should ideally just forbid this.

If we do so, a lot of the cases where borrowing code needs to do "all longer lifetimes" can be cleaned up as well.

[Manishearth]

Honestly, I think the best route might be to fully expand lifetimes during lowering.

I've already got the validity check in #437

[Manishearth]

Fixed by 7687c0f