Add Copy to ArrayVec enabled by a feature

[pYtoner]

Completes #32.

Unfortunately, because trait bounds other than Sized are not allowed for const functions the new_const function is not available if the copy feature is enabled.

The whole idea behind the whole thing is that there is no way to construct an ArrayVec<T> that isn't T: Copy if the copy feature is enabled.

[bluss]

Hi, this is a non-additive feature and the general guidelines for libraries are that features have to be additive, so this can not be included in this way, unfortunately. There should be a separate ArrayVecCopy type for this use case, and it should use the ArrayVecImpl trait (and flesh it out) to share as much code as possible.

[HughHoyland]

That's regrettable. I would totally expect a stack-allocateable ArrayVec to be Copy. That's the main use case of fixed-size stack-allocateable Vec for me.