roll: third_party/pigweed 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'.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/322562
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 7ef8e44105f924b1ebbeded39941bceda1feb62d

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: d4e45ece255237..7ef8e44105f924
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8702111458229239681
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I01fbc8c51326de33a87474b111ec8413a937f8b8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/328392
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index d4e45ec..7ef8e44 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d4e45ece2552376f02577a3858c592fd64a87ee1
+Subproject commit 7ef8e44105f924b1ebbeded39941bceda1feb62d