[roll] Roll fuchsia [fidl][cpp] Move function bodies out of natural & driver headers

(this is a reland of I7085c18fe75c3d14c2ffbd90c3fc7437fefaf9f4)

This moves almost all function bodies out of generated bindings headers
for natural bindings and both flavours of driver transport bindings.
This means that they're only evaluated and compiled once per build
(modulo toolchain, subbuild, etc). Because these headers are very widely
included this has an impact on the build time for many compilation
units. The remaining function bodies mostly just constexpr functions
associated with bits and enum types.

I compared compile times for one of the slower targets in our C++ build,
src/connectivity/bluetooth/hci/transport/uart/bt_transport_uart.cc and
it reduced the compile time (averaged measured with hyperfine) from an
average of 40.5s to 19.0s, a 53% speedup.

There were some microbenchmark regressions but they were largely in the
setup and teardown parts steps and in benchmarks that don't represent
what's heavily used in production.

We're requesting an larger compile machine for benchmark FIDL files
because it was causing issues in one build.

Original-Bug: 361634289
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1265227
Original-Revision: a3d2e22d4673c44f65fd1fe1d28de4c9109aa564
GitOrigin-RevId: 4a94783205161b72adad4bb542d6a433db5acb19
Change-Id: I44ec27d76124e977bfd1ecb29021c3e5d26c58f2