Implement Index and IndexMut traits for ArrayVec

[niluxv]

I didn't have time to add tests to it yet, but this should implement Index<I> and IndexMut<I> where I: SliceIndex<[T]>.

Fixes #190

[niluxv]

Instead of this implementation (self.get[_mut](index).unwrap()) we could also use

self.deref().index[_mut](index)

Maybe that gives better error messages in some situations? I don't know...

[bluss]

Looks to be in the right direction, that's nice - I'll be a bit unreachable during the summer, but back in a bit

[niluxv]

No problem.

I changed the implementation as described above because it indeed gives much better error messages on out of bounds errors, and added some tests.

[bluss]

Thanks, yes not regressing the panic messages is certainly a requirement. One could also debate if I: SliceIndex or if [T]: Index<I> is the trait bound we should use (the latter being yet more general), but it seems fine as it is now.

We need the corresponding change for ArrayString too, but it can be a different PR.

[niluxv]

So I changed the trait bounds as you suggested and just now added implementations to ArrayString analogous to those for ArrayVec.
I also removed the documentation I initially added to the Index/IndexMut implementation since the comment on panicing needn't be true for all instantiations of the generic parameter.