[SR-3396] `reversed` for sequences and forward collections and should return a `ReversedRandomAccessCollection<[Iterator.Element]>`

Previous ID	SR-3396
Radar	rdar://problem/21098134
Original Reporter	@airspeedswift
Type	Improvement

Attachment: Download

Votes	0
Component/s	Standard Library
Labels	Improvement, AffectsABI, StarterProposal
Assignee	None
Priority	Medium

md5: 34c0ebf025eb58406a81d44f15afed48

Issue Description:

Currently calling reversed() on a bi-directional and random-access collections returns a ReversedCollection and ReversedRandomAccessCollection respectively. This is because the view can be placed over the underlying collection at no cost – it's just a question of transforming the index appropriately and then accessing the underlying base collection.

For sequences and forward-only collections, however, this can't be done. So for these, the contents are read into an array and then that array is in-place reversed. But the second step is unnecessary – the array, once created, could just be returned wrapped in a ReversedRandomAccessCollection. As well as efficiency, this would have the benefit of making these methods more consistent across the board in what they return.