der: add impl OwnedToRef for UintRef and IntRef

[dishmaker]

Edit: This might be useful. But looks like AsUintRef solves the problem.

impl<U> RsaPrivateKey<U>
where
    U: AsUintRef,
{
    pub fn public_key(&'u self) -> RsaPublicKeyRef<'u> {
        RsaPublicKeyRef {
            modulus: self.modulus.as_uint_ref(),
            public_exponent: self.public_exponent.as_uint_ref(),
        }
    }
}

5d46ec0#diff-65c869a52db3faff4591aacf0874b552096a0ac7ad0bd4b0908c7fb82ada6c98R90

[dishmaker]

Ah, ok, AsUintRef solves the problem