[roll third_party/pigweed] pw_containers: InlineVarLenEntryQueue::try_push() function

- Add try_push(), which complements push_overwrite(), but drops new
  entries instead of overwriting old.
- Add max_size() function, which returns the maximum number of (empty)
  entries.
- Docs updates.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/213612
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: aee408cfeb5522c..ff81e38b61cc14b
Roller-URL: https://ci.chromium.org/b/8746116347531301489
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I79d27c77786b023847fcbbcb5d2d2f8cbe872e9f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/213672
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
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 aee408c..ff81e38 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit aee408cfeb5522ce5cf1e736873ef3868e737c36
+Subproject commit ff81e38b61cc14b3f95e68e769b60e3b66b1c4c6