Inherent const impl

[oli-obk]

Some constifications are annoying because we need to repeat T: Trait bounds from an impl block on the individual constified const fns as T: [const] Trait. We've brainstormed solutions before, and one would be to have separate const impl blocks or sth. However the final syntax will look, I decided to just impl this syntax and either have sth nice on nightly to work with or at least move the discussion along.

Also interacts with the discussion around impl const Trait for Type vs const impl Trait for Type, as we may want to use the latter to keep inherent and trait impls in sync (unless we come up with even another scheme).

• rustdoc + tests
• macro stability /regression tests

r? @fee1-dead

cc @traviscross @rust-lang/project-const-traits