Remove Drop for ArrayDeque and implement Copy

[stjepangolemac]

Closes #27.

I went with a minimal change set to allow ArrayDeque to implement Copy. Iterator types still implement Drop.

Removed test was failing so I removed it, for now. I see several possible outcomes:

1. This gets merged as is (in case Drop is not too important for common use cases, it certainly isn't for me)
2. We put ArrayDeque Drop and Copy implementations behind features, one excluding the other (what to do with this test then?
3. This PR gets rejected (in which case I'll keep the fork around)