Get `UnsafeRawBufferPointer(s)` protocols

[Karl]

@johannesweiss

Data doesn't guarantee that the memory is bound to UInt8 -- in fact it specifically allows the user to bind it to whatever they need -- and hence can't implement withContiguousStorageIfAvailable.

I think that was relaxed recently:

@Andrew_Trick

As of Feb 5, 2022, it is reasonably safe to implement Foundation.Data's withContiguousStorageIfAvailable by using withMemoryRebound: SE-0333 Expand usability of withMemoryRebound