[Pitch] Restrict pointer conversion to C interoperability

There has been discussion of async versions of withUnsafeMutableBufferPointer functions (always with a bound). I would prefer to rely on something like BufferView and lifetime dependencies, and that will provide the same speed as pointer+length in nearly every case. (It would also have an unsafe hatch for C interop.)

In every case there is a verbose solution (one of the withUnsafe[Buffer]Pointer), but I hope we'll be able to be smarter and point to the attribute when it is relevant.