Rework reservation impls into an `Ambiguous` trait

[nikomatsakis]

From this comment in #90104:

As a follow-up PR I would like to remove reservation impls and replace them with an (perma-unstable) Ambiguous trait. Then we could replace a reservation impl like

impl Foo for T
where
    T: Ambiguous,
{
}

We have to double check if this will work though!

[nikomatsakis]

cc @spastorino

[the8472]

Shouldn't we dog-food negative impls? Or do you also mean removing the feature entirely and not just from std?