ContiguousCollection protocol revisited (#990)
ContiguousCollection protocol revisited (#990)

* First cut

* Switch to vDSP_vadd

* reindent

* Typo in vDSP_vadd

* Update proposals/NNNN-contiguous-collection.md

Co-Authored-By: airspeedswift <f6358031b01ba395cdd151dcadd901fffc88c517@users.noreply.github.com>

* Add default implementation of withContiguousStorageIfAvailable

* Flesh out alternatives section

* Array caveats

* Update NNNN-contiguous-collection.md

apple/swift-evolution,DevAndArtist/swift-evolution,ddunbar/swift-evolution,hartbit/swift-evolution,hartbit/swift-evolution,anandabits/swift-evolution,abertelrud/swift-evolution,apple/swift-evolution,DevAndArtist/swift-evolution,anandabits/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution,natecook1000/swift-evolution,Nirma/swift-evolution,ddunbar/swift-evolution,Nirma/swift-evolution