Implement `Allocator` for `Bump`, not `&Bump`

[fitzgen]

Bump does not implement Allocator but &Bump does. So to get a &dyn Allocator you'd have to take a reference to the reference:

&&bump_arena as &dyn Allocator

That being said, I think the impl Allocator for &Bump could be changed to impl Allocator for Bump so you wouldn't have that double indirection.

Originally posted by @bluurryy in #280

[bluurryy]

I don't think this would be a breaking change. How is it breaking?

EDIT: since there is a blanket impl Allocator for all &T where T: Allocator

[fitzgen]

EDIT: since there is a blanket impl Allocator for all &T where T: Allocator

Yep. Although maybe removing the explicit import for &Bump and allowing the blanket impl to fill that in means this wouldn't be a breaking change. Not totally sure.