pw_multibuf: Reorganize v2 types

MultiBuf.v2 previously made some choices that could be improved:
1. It put too many secondary types in the 'pw' namespace.
2. It followed v1's style of using a 'multibuf_impl' namespace instead
   of 'internal'.

Change-Id: Ief0f56bd01f54213004eab256144964c62ea9198
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/322562
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Lloyd Pique <lpique@google.com>
Commit-Queue: Aaron Green <aarongreen@google.com>