Implement assert_trait_sub_any

[Nadrieril]

It is constructed quite straightforwardly from assert_impl_any by
quantifying using a generic function. Sadly it can't actually reuse
assert_impl_any directly because of "using parameter from outer
function" error.

[nvzqz]

Looks great, thank you so much for this! Just one small change, please. 🙂

I was scratching my head forever on how to get this to work. I started thinking thinking up all sorts of shenanigans for this that would require trait impl specialization.

When I asked @killercup how to do this, he was thinking of doing something like:

trait A {
    const VALUE: usize;
}

default impl<T> A for T {
    const VALUE: usize = 0;
}

trait B {}

impl<T: B> A for T {
    const VALUE: usize = 1;
}

And then somehow adding up VALUE and checking if the result is greater than 0.

[Nadrieril]

Well, if you're adding up integers you could just as well || booleans together. And the wrapper trick allows for a limited form of emulating specialization. So this idea isn't too different

[nvzqz]

Ah good point. Unfortunately || doesn't work in const yet, but | does.