Use <[_; N]>::each_ref() when available

[martinthomson]

There are a few places where we construct a fixed size array of objects, then run that through map() in order to produce another fixed size array. For example:

        let bit_ctx: [_; 3] = top_ctx
            .iter()
            .map(|ctx| ctx.narrow(&format!("bit{}", bit)))
            .collect::<Vec<_>>()
            .try_into()
            .unwrap();

This is super awkward as you have to iterate the slice, map, collect into a Vec<_>, then use try_into() to maybe create the array, then unwrap that. This would be cleaner, once each_ref() becomes available in mainline rust.

        let bit_ctx = top_ctx.each_ref().map(|ctx| ctx.refine(format!("bit{}", bit)));

[akoshelev]

Looks like things are moving to stabilization: rust-lang/rust#76118

rust-lang/rust#103522