Support `each_ref` and `each_mut` in `[T; N]` in constant expressions.

[bjoernager]

Tracking issue: #133289

The methods <[T; N]>::each_ref and <[T; N]>::each_mut can easily be reimplemented to allow marking them with the const specifier.

This specific implementation takes a different approach than the original as to avoid using iterators (which are illegal in constant expressions).

[rustbot]

r? @jhpratt

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[bjoernager]

r? libs-api

@rustbot label +T-libs-api -T-libs

[bjoernager]

Should lifetimes be explicit here or can the compiler still deduce that they derive from that of self?

[bjoernager]

E.g.:

pub const fn each_ref<'a>(&'a self) -> [&'a T; N];

[jhpratt]

@bors r+

[bors]

:pushpin: Commit 7c799c3e0d0c5e34126511467d62e1b654f41d3e has been approved by jhpratt

It is now in the queue for this repository.