Auto merge of #148434 - oli-obk:inherent-const-impl, r=fee1-dead

Inherent const impl

Some constifications are annoying because we need to repeat `T: Trait` bounds from an impl block on the individual constified `const fn`s 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`

Author: bors