More general PartialEq implementation for Option

Currently, only the case where T == U is covered. Presumably this is because of derive limitations, since this impl is indeed derived in libcore.

I believe it should be possible and correct to provide the following impl:

impl<T, U> PartialEq<Option<U>> for Option<T> where T: PartialEq<U>, U: PartialEq<T> {
    fn eq(&self, rhs: &Option<U>) -> bool {
        match (self, rhs) {
            (&Some(ref lhs), &Some(ref rhs)) if lhs == rhs => true,
            (&None, &None) => true,
            _ => false
        }
    }
}

Start with the Option PartialEq implementation in libcore and the linked Option::eq documentation. Compare the existing same-type case with the proposed cross-type implementation, then check whether Rust's trait rules permit it. Done means the supported PartialEq behavior is clearly established and covered by the relevant core tests.