* Port v8 fuzzer to the new framework
This commit aims to improve the support for the new "meta" differential
fuzzer added in #4515 by ensuring that all existing differential fuzzing
is migrated to this new fuzzer. This PR includes features such as:
* The V8 differential execution is migrated to the new framework.
* `Config::set_differential_config` no longer force-disables wasm
features, instead allowing them to be enabled as per the fuzz input.
* `DiffInstance::{hash, hash}` was replaced with
`DiffInstance::get_{memory,global}` to allow more fine-grained
assertions.
* Support for `FuncRef` and `ExternRef` have been added to `DiffValue`
and `DiffValueType`. For now though generating an arbitrary
`ExternRef` and `FuncRef` simply generates a null value.
* Arbitrary `DiffValue::{F32,F64}` values are guaranteed to use
canonical NaN representations to fix an issue with v8 where with the
v8 engine we can't communicate non-canonical NaN values through JS.
* `DiffEngine::evaluate` allows "successful failure" for cases where
engines can't support that particular invocation, for example v8 can't
support `v128` arguments or return values.
* Smoke tests were added for each engine to ensure that a simple wasm
module works at PR-time.
* Statistics printed from the main fuzzer now include percentage-rates
for chosen engines as well as percentage rates for styles-of-module.
There's also a few small refactorings here and there but mostly just
things I saw along the way.
* Update the fuzzing README
This commit removes the `differential_spec` fuzz target for now,
although this removal is intended to be temporary. We have #3251 to
track re-enabling the spec interpreter in a way that it won't time out,
and additionally the spec interpreter is also failing to build with
ocaml on oss-fuzz so that will also need to be investigated when
re-enabling.
The WebAssembly spec interpreter is written in OCaml and the new crate
uses `ocaml-interop` along with a small OCaml wrapper to interpret Wasm
modules in-process. The build process for this crate is currently
Linux-specific: it requires several OCaml packages (e.g. `apt install -y
ocaml-nox ocamlbuild`) as well as `make`, `cp`, and `ar`.
