roll: third_party/pigweed pw_containers: Multiple container example

It is possible to add intrusive items to multiple containers, but only
if those containers have disjoint value_types. This isn't entirely
intuitive, so this CL adds examples, unit tests, and docs to make it
easier.

Additionally, in order facilitate the aforementioned unit tests, the
base type for map items has been unified into `pw::IntrusiveMapItem`,
and the intrusive item type used by `CheckItemType` has been renamed
to `pw::containers::internal::IntrusiveItem`.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/237472
Original-Revision: 819dd2ceb430eae53909bea7d5a23c7743ef0fc2

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: cfa4da0ff6f50b..819dd2ceb430ea
Roller-URL: https://ci.chromium.org/b/8735780313316340593
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id366b2bf135b2e7262685f9a9d3ba581a73ba031
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/237907
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Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index cfa4da0..819dd2c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit cfa4da0ff6f50b81e21c3343312e15caf41a2bdf
+Subproject commit 819dd2ceb430eae53909bea7d5a23c7743ef0fc2