[roll] Roll fuchsia [fidl][rust] Refactor decoding layer

This change started out with adding a lifetime to the `Decoder` trait to
reduce the amount of unsafe code we have to write. That gradually spread
to a few other major changes as well:

- `Decoder` no longer has any safety requirements, and can be
  implemented without unsafe.
- The `Unconstrainted` trait has been removed. It was a nice shortcut
  for implementation, but was confusing since it shouldn't be used for
  bounds.
- A new `AsDecoder` trait was added, finally allowing us to share the
  basic implementations of `Decoder` between FIDL, DF, and FDomain.
- Wire types have been moved into a `wire` module for clarity. In the
  future, I'd like to further remove "Wire" from their names and refer
  to them via `wire::`. This matches the structure of the generated
  bindings.
- Device-side tests have been fixed and re-enabled for fidl_next_bind
  and fidl_next_codec.
- The protocol layer now has a `Body` wrapper instead of using the
  transport backend's receive buffer directly. This is a knock-on effect
  from `AsDecoder` and will enable the protocol layer to check for
  flexible errors.
- References to `Constrained` in bounds have been merged into `Wire` for
  clarity.
- Codec layer impls for builtin types have been moved into one file.
  This keeps all of the related implementations lexically close to each
  other.
- A few functions and methods have been renamed for clarity, including
  `decode_owned`.
- `WirePointer` now has a safer API that helps guard against pointing to
  memory that it outlives.
- Some additional `PartialEq` impls have been added for ergonomics and
  to make writing tests easier.
- Several helper functions have been replaced with direct
  implementations. This is mostly because we have more ergonomic APIs.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1505016
Original-Revision: 8f685ac0a2b9c796682e852b6059f7f9dc3c3db9
GitOrigin-RevId: 0f95e0b477199f497795f1052dc2b7acd24ac991
Change-Id: I4a077d6f1814d35b5b749e53b402dadd8d4254d2