[roll third_party/pigweed] pw_stream: Add a ReaderWriter base class

Add a `class ReaderWriter` base class that combines interfaces of
pw::stream::Reader/Writer.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48462

third_party/pigweed Rolled-Commits: d69309b5a7ffc2a..78d0730d24cfb89
Roller-URL: https://ci.chromium.org/b/8844684391187944225
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8844684391187944225
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I455cfe5901e87d7f460e2190edacfe54c3f91f5b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48668
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index d69309b..78d0730 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d69309b5a7ffc2afbbd46afbf68f0b818ea417a3
+Subproject commit 78d0730d24cfb893bc8cd0cfad9a5521fa55feae