Add something like `num_traits` to STD, or move it into std entirely

jhpratt:

just to make generic implementations easier

For that, I wonder how far you could get with something like https://github.com/rust-lang/rfcs/pull/3686#discussion_r1755464148 instead.

Being able to have both

impl<N> TheTrait for u<N> { … }
impl<N> TheTrait for i<N> { … }

is way better than doing that via a trait, because it's no longer a top-level blanket.

As soon as you have

impl<T: std::sealed::Whatever> TheTrait for T { … }

now it's more annoying to implement it for other things too.