[roll third_party/pigweed] pw_containers: Make List::Items movable,

Since moving items is an O(n) operation, this CL also updates the docs
to discuss the complexity of various list operations.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149030

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: b072fb193602e56..384784322158075
Roller-URL: https://ci.chromium.org/b/8779376489794346961
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1643d5f558b695105960bceebd14303d4668500c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/150255
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 b072fb1..3847843 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b072fb193602e56585efe7ba13c97cfdd0144437
+Subproject commit 38478432215807547d30b1e5da7a8428df5dd810