Rollup merge of #73862 - oli-obk:const_array_to_slice, r=RalfJung

Rollup merge of #73862 - oli-obk:const_array_to_slice, r=RalfJung

Stabilize casts and coercions to `&[T]` in const fn

Part of #64992

There was never a reason to not stabilize this, we just accidentally prevented them when we implemented the `min_const_fn` feature that gave us `const fn` on stable. This PR stabilizes these casts (which are already stable in `const` outside `const fn`), while keeping all other unsizing casts (so `T` -> `dyn Trait`) unstable within const fn.
These casts have no forward compatibility concerns with any future features for const eval and users were able to use them under the `const_fn` feature gate already since at least the miri merger, possibly longer.

r? @rust-lang/lang