roll: third_party/pigweed pw_multibuf: Move internal iterator headers to internal/

The chunk and byte iterator headers never need to be included by users.
Move them to internal/, combine their build targets, and make the new
target private.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/303217
Original-Revision: 815a7345a28fd3c365852c2ebc1d421f75a45da4

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 632e84a744694c..815a7345a28fd3
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8709932362240100305
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0f39756a82b4c7bce7fbe4c1fb42346c329feb47
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/303276
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 632e84a..815a734 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 632e84a744694c7d1f089e256236236a6b47a877
+Subproject commit 815a7345a28fd3c365852c2ebc1d421f75a45da4