`u32` as a second fallback type

[toc]

@scottmcm

I think it's much harder to argue that only usize should be supported for indexing a [_; 16] , though. And especially for LUTs like [_; 256] , indexing by u8 is really elegant.

I would love this, but when I have seen it mentioned in the past the kind of "shortest path" fix would need some kind of default type hinting. The (technically minor) breakage for all code that resembles

let i = 2;
let x = xs[i];

is pretty severe. So it would be nice in that case if the type of i were inferred to be usize.