Add a way to append another array

[jD91mZM2]

I'm not sure, but I suspect having an append function that consumes another arrayvec and ptr::copys over all elements at once would be faster than using extending an iterator which ptr::writes per-item.

[bluss]

Ideally extend shouldn't ptr::write per item, it should optimize better than that, but I suspect the IntoIter implementation is putting limits on how well that can be done.

PR welcome, please include tests.