Currently, the Queue will never reuse it's full capacity if you call `pop_front`, even if you empty it before pushing more items.
With this change, if you empty the Queue with `pop_front`, then the offset will be set back to the start of the underlying array when you pop the last item.
Future pushes will then reuse the already-allocated--but now empty--space.
We were previously using array_slice to get the storage
that we were copying the new elements into, using the current
length as the offset: `copy(data[len:], ..elems)`
However, array_slice returns a slice over `data[0:len]` -- we
were using it as if it was `data[0:cap]`.
Add array_cap_slice that does this instead. :^)
