flatten/unflatten functionality

[Dzuchun]

I wonder if there's a way to flatten/unflatten a two-dimensional array. I think that as long as dimensions are properly enforced with the bounds, it should be safe to simply cast between them, since GenericArray has the same layout as core array.

It is trivial to implement anywhere, but I feel like it should be a part of this crate.

[Dzuchun]

Looks good to me!
(should we close the issue, then?)

[novacrazy]

I haven't published it yet, was attempting some other things while I was at it. The lack of const given it's implemented via a trait is a non-issue, then?

[Dzuchun]

Yeah, I used const in the gist just because I could.

These methods not being const is consistent with rest of the crate, so that's ok.

[novacrazy]

Published in v1.1.1