AsRef and AsMut between T and [T; 1]

[cole-miller]

Like what @CAD97 suggested here, right?

Min min specialization: replaceable impls?

To hopefully bypass syntax bikeshed, I'm using an attribute #[replaceable] to mark replaceable impls here. The hope is that #[replaceable] should be able to straightforwardly evolve forwards-compatibly to be expressed in terms of a proc macro attribute on top of today's feature(min_specialization) but be easier to show sound and stabilize sooner. In short: allow writing #[replaceable] impl<T, ⋯> Trait for T { ⋯ } where the blanket impl is "replaceable" by a specific implementation. I will use …