RFC: Feature/newtype

[cehteh]

I anticipate the change from the type alias to a newtype implementation here. Maybe this qualifies/requires a major version bump (I suspect this is still binary compatible as its a transparent repr).

A possible alternative would be to implement 'with()/with_mut()' functions as toplevel crate functions (I wont necessary like that, but ymmv). Keeping them in the Erasable trait won't be totally bad as well.