Mutable arrays

[otrho]

#18 and #424 introduced const arrays but mutable arrays still need to be done

A part of this may be to introduce the IndexMut trait and to call it (instead of the Index trait) for mutable values for types other than static arrays. See #429

[mohammadfawaz]

@otrho do we still need this if we're going to have Vec?

[adlerjohn]

Ref: #1118

[otrho]

This would be having

let mut counters: [u64; NUM_COUNTERS] = [0; NUM_COUNTERS];
...
counters[THING_IDX] += 1;
...

So, I would say yeah, it'd be useful, but low priority.

[mohammadfawaz]

@tritao looks like #429 already exists to track Index and IndexMut. Will close this again in favor of that one. I also restored the old title.