Implement conversion traits between arrays and uniform tuples.

[SimonSapin]

AsRef, AsMut, and Into in both direction between [T; N] and (T, T, …), for N from 0 to 32.

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[SimonSapin]

I don’t have a strong motivation for this, it just seemed neat. Arrays and uniform tuples are interchangeable to some extent, and I believe their memory representation is the same. (Is that guaranteed to always be the case?)

[alexcrichton]

This was previously proposed in #30736, to which the libs team concluded that the full generalization is very var out, so if there's not a strong motivation to have this in the meantime we'd prefer to not land.

[petrochenkov]

cc rust-lang/rfcs#1397

[SimonSapin]

Fair enough.