[roll third_party/pigweed] pw_allocator: Add extra bytes to Block

The CL adds a new template parameter, kNumExtraBytes, to `Block`, and
refactors the existing parameters. The new parameter can be used to
reserve space in the block's header that can be used to store and fetch
allocation metadata.

Additionally, the `kMaxSize` parameter has been dropped and replaced by
a parameter for `kNumFlags`. This allows better elision of flag-related
code when it is not needed, and encourages downstream projects to define
fewer block types rather than one for each allocation pool size.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/185954

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 877273458e68466..fe8e3d2b5eda051
Roller-URL: https://ci.chromium.org/b/8759715729657905073
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I61dd49688795e9c0ee9597e97c62c0abde82f336
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/186936
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 8772734..fe8e3d2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 877273458e68466949a2f68373150ae3d7bf6a3a
+Subproject commit fe8e3d2b5eda0518a98db2ab346d4e450b530a73