publicly export constraint::Constraint but seal it

[changhe3]

Currently you cannot meaningfully implement trait or inherent implementations with respect to Proxy<T, P> since you cannot name P: Constraint<T>. By sealing it, only the crate itself can directly implement the Constraint trait, but everyone else can write things like:

impl<T, P: Constraint<T>> SomeTrait<Proxy<T, P>> for MyStruct {}