[roll third_party/pigweed] pw_multibuf: Add module and Chunk implementation

This is the beginnings of the implementation of SEED-0109.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/173951

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 24eef22e00e02fa..fb640c84662c744
Roller-URL: https://ci.chromium.org/b/8766401441310157201
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ibe9fa660e25a21a7489b09bdd082fd6f47b81ecd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/177462
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 24eef22..fb640c8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 24eef22e00e02faaac56249560069e52a26fd0aa
+Subproject commit fb640c84662c7444f3efb41c5a7abc5fd4091f3b