Relax implicit `Sized` bound on some blanket impls

[E-Mans-Application]

The blanket implementations of CanonicalSerialize and CanonicalDeserialize have an implicit bound T: Sized

As a result, Cow<'a, [T]> (for example) does not implement those traits even if T does.

The bound may be relaxed to address this case:

impl<'a, T: ?Sized + CanonicalSerialize + ToOwned> CanonicalSerialize for Cow<'a, T> {
    //
}

The same can apply to Rc<T> and Arc<T>.