Consider restricting implementations of SizeLimit

[dtolnay]

I don't think 1.0 should have a SizeLimit trait that allows external implementations until somebody proposes a really good use case for it. In serde_json we restrict implementations by using a private supertrait:

pub trait SizeLimit: private::Sealed {
    #[doc(hidden)]
    all the methods
}

mod private {
    pub trait Sealed {}
    impl Sealed for super::Infinite {}
    impl Sealed for super::Bounded {}
}

This allows adding / changing methods in the future as necessary without a major version bump.