Rollup merge of #130164 - RalfJung:const_ptr_as_ref, r=dtolnay

move some const fn out of the const_ptr_as_ref feature

When a `const fn` is still `#[unstable]`, it should generally use the same feature to track its regular stability and const-stability. Then when that feature moves towards stabilization we can decide whether the const-ness can be stabilized as well, or whether it should be moved into a new feature.

Also, functions like `ptr::as_ref` (which returns an `Option<&mut T>`) require `is_null`, which is tricky and blocked on some design concerns (see #74939). So move those to the is_null feature gate, as they should be stabilized together with `ptr.is_null()`.

Affects #91822, #122034, #75402, rust-lang/rust#74939