Generalise lifetimes for binop implementations

[Jacobgarm]

The current implementations of the binary operator traits for OrderedFloat and NotNan expect both operands to have the same lifetime (or at least not general lifetimes). This is fine in the majority of cases, but fails when working with higher order trait bounds. As an example, the following code doesn't compile on master:

fn foo<T>(x: T) -> T
where
    for<'a, 'b> &'a T: Add<&'b T, Output = T>,
{
    &x + &x
}

fn main() {
    foo(OrderedFloat::from(2.0));
}

The change is to manually speficify the type of the left hand side in the implementation, rather than using Self. This makes the above example compile.

[mbrubeck]

Thanks!