Implement IndexMut

[SUPERCILEX]

Allows syntax like received[id] = true;

[james7132]

I don't think it's possible to implement this as the Index implementation uses borrows on constants &true and &false. Providing a &mut bool return value is likely impossible.

[SUPERCILEX]

Pretty sure you just need a wrapper type. Don't return bool, return some wrapper type that implements DerefMut for bool. That way you can call set in the deref implementation.

[james7132]

IndexMut relies on Index's associated type. We'd need to do the same for Index and I'm not sure if that's desirable from an API design perspective.

[SUPERCILEX]

Ah shoot, forgot about that. I think it should mostly work? Off the top of my head, if a user stores the result of bitset[blah] in something that uses type inference, things might get weird. They'll probably store the wrapper type instead of the bool. Then again, they would have been storing &bools before, so maybe that's already broken and it doesn't matter?