This fixes a build issue with the most recent version of `libfuzzer-sys`
where the prior version of the `fuzz_target!` macro didn't need the
crate as an explicit dependency but the latest version does.
* Remove unused regalloc2-test crate
This code doesn't build, and Chris says it's "a really old harness that
existed prior to building the fuzzing and was used mainly to profile and
get stats before integration with Cranelift".
* Re-export libfuzzer/arbitrary from fuzzing module
This avoids needing to keep dependencies on `arbitrary` in sync across
the three different Cargo.toml files in this project.
However, before version 0.4.2, libfuzzer-sys only supported using its
macros if it was available at the top-level `libfuzzer_sys` path, which
breaks when re-exporting it. So I'm upgrading to that version (or the
newest patch release of it).
Upgrading libfuzzer-sys in turn brings in the 1.0 release of the
arbitrary crate, with a minor API change along the way.
