[roll third_party/pigweed] pw_{hdlc,log,log_rpc,span}: Return pointer instead of iterator

Replace begin() with data() where applicable for span.
Original-Bug: b/239856986

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/111591

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: b9bc4e28917ea9f..a81ed7459b3b454
Roller-URL: https://ci.chromium.org/b/8802285724109734577
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7af79f262acd3609fdda54da627590d209eacd85
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/111612
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index b9bc4e2..a81ed74 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b9bc4e28917ea9fdcf1fe5acb81dfb1113480bb2
+Subproject commit a81ed7459b3b4547947ce2db5c6764d4f670c4b1