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!

Change-Id: I9819eb23c9cdf745691ba15b20648b824620cf68
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/230892
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Reviewed-by: Taylor Cramer <cramertj@google.com>