Spanify CopyOnWriteBuffer

Detach CopyOnWriteBuffer implementation from Buffer by creating a dedicated small type that has non-mutable capacity and doesn't have size. Each CoWBuffer object sharing the same buffer has its own members that specify which memory can be safe to write or read.
Delegate all pointer arithmetic to ArrayView silencing unsafe-buffer warnings.

Bug: webrtc:478086887
Change-Id: I2ee0c0fce6c4264c5d44e0f227908fc9c9afadc7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444702
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46885}