roll: pigweed, pw_toolchain: pw_multibuf: Restructure ChunkIterable

MultiBuf is a byte-oriented view of a list of chunks, and this change
structures MultiBuf accordingly. This replaces the ChunkIterable class
with a private MultiBufChunks base class that provides the
Chunk-oriented view for MultiBuf.

Restructuring this way fixes an issue where MultiBuf data can be
modified from a const reference. MultiBuf originally returned a const
ChunkIterable, but since return values are copied, this becomes a
non-const ChunkIterable, giving mutable access to the multibuf.

  const MultiBuf& const_mb = mb;
  auto chunk_iterable = cmb.Chunks();  // chunk_iterable is non-const
  iterable.front()[0] = std::byte();  // uh oh, chunk data is mutable!

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/230892

https://pigweed.googlesource.com/pigweed/pigweed
pigweed, pw_toolchain Rolled-Commits: 03da4a376d12ab8..7e7c141c8808200
Roller-URL: https://ci.chromium.org/b/8739051696451980865
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8af7a904b5cd447e9e4b483987c21f057851671f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/230736
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index a96f546..0df9a2b 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -48,8 +48,8 @@
     module_name = "pigweed",
     # ROLL: Warning: this entry is automatically updated.
     # ROLL: Last updated 2024-08-20.
-    # ROLL: By https://cr-buildbucket.appspot.com/build/8739052084952556721.
-    commit = "03da4a376d12ab8173f250a14f703ae8a1571d3c",
+    # ROLL: By https://cr-buildbucket.appspot.com/build/8739051696451980865.
+    commit = "7e7c141c88082004fa8ae3682bf37f3a8229af11",
     remote = "https://pigweed.googlesource.com/pigweed/pigweed",
 )
 
@@ -57,8 +57,8 @@
     module_name = "pw_toolchain",
     # ROLL: Warning: this entry is automatically updated.
     # ROLL: Last updated 2024-08-20.
-    # ROLL: By https://cr-buildbucket.appspot.com/build/8739052084952556721.
-    commit = "03da4a376d12ab8173f250a14f703ae8a1571d3c",
+    # ROLL: By https://cr-buildbucket.appspot.com/build/8739051696451980865.
+    commit = "7e7c141c88082004fa8ae3682bf37f3a8229af11",
     remote = "https://pigweed.googlesource.com/pigweed/pigweed",
     strip_prefix = "pw_toolchain_bazel",
 )