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