[wire] Spanify BufferConsumer and it's callsites.

- This change spanifies the APIs provided via BufferConsumer.
  This is an Dawn internal helper class that needs to be
  spanified to facilitate more spanification of the Dawn
  wire API.
- Removes BufferConsumer_impl.h since separating the templates
  and the implementations of them was making it harder to
  update them, and the implementations aren't too big, so the
  compilation gains should be marginal.
- A lot of the wire transfer structs needed volatile
  constructors to satisfy std::span iterator requirements.
  Since we don't actually expect these constructors to be
  called (they are needed for some algorithms that need to
  copy or duplicate elements), we just trivially implement
  them with a DAWN_UNREACHABLE().

Bug: 528027992
Change-Id: I60cfa340df03104057c51c292776942fd17a897f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/320696
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Auto-Submit: Loko Kung <lokokung@google.com>