Allow array indexing

[schoen]

Project

core

Describe the feature

Maybe this is tricky in terms of the type system, but I think it would be great to able to do

let a: [u8; 4] = [0, 17, 255, 23];
let b: u8 = a[3];

and similar. Right now you have to actually destructure the whole array in order to get individual values. For example, p2ms.simf does

    let [pk1, pk2, pk3]: [Pubkey; 3] = pks;
    let [sig1, sig2, sig3]: [Option<Signature>; 3] = maybe_sigs;

and you can't conveniently refer to the value at position i at all. I don't think there is an architectural reason that this is meant to be prohibited.

(Among other things, this means that it's hard-to-impossible to loop over an array with for_while, although you can differently loop over an array with array_fold.)