[roll] Roll fuchsia [packet] Simplify buffer trait hierarchy

`packet`'s family of buffer traits bottom out at two fundamental
flavors: fragmented buffers and contiguous buffers. Fragmented buffers
are those whose bodies may be discontiguous in memory, while contiguous
buffers are those whose bodies are guaranteed to be contiguous.

Previously, we used extra marker traits and blanket impls in order to
reduce the amount of code we needed to write. However, this introduced
unnecessary complexity in the API that only made the API more convoluted
for users to understand.

In this commit, we make the following changes:
- All buffer traits bottom out at `FragmentedBuffer`
- `ContiguousBuffer` is a sub-trait of `FragmentedBuffer`
- Impls which were previously achieved via blanket impls aided by the
  `ContiguousBuffer[Mut]Impl` marker traits are now implemented by hand
  with the aid of a macro; those marker traits are removed

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/862776
Original-Revision: fd4abd876d815b1385ac3fad3c0b99d430a67ff1
GitOrigin-RevId: ead7fe9262bf6989edf983831e79ea3a8425f022
Change-Id: I7ab19b5cc10e91acad7c74b93263f72471db7679