[roll third_party/pigweed] pw_stream: Add mpsc_stream

This CL adds multi-producer, single-consumer memory stream. This allows
writing from one or more threads and reading from another.

Original-Bug: b/287666300
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/135960

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: a2151554e943ec3..eaaf71dedd50cbf
Roller-URL: https://ci.chromium.org/b/8773126198090583041
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic7a3425e0a1f43dbf30c0f6bf4a0fc368a5127a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/164850
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 a215155..eaaf71d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a2151554e943ec33a6ed8a6bd12ad03afcb1426b
+Subproject commit eaaf71dedd50cbfe6e74984b6b7d7521fa3efee4