Port v8 fuzzer to the new framework (#4739)

* 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

fuzz/README.md

@@ -37,10 +37,8 @@ At the time of writing, we have the following fuzz targets:
 * `differential`: Generate a Wasm module, evaluate each exported function
   with random inputs, and check that Wasmtime returns the same results as a
   choice of another engine: the Wasm spec interpreter (see the
-  `wasm-spec-interpreter` crate), the `wasmi` interpreter, or Wasmtime itself
-  run with a different configuration.
-* `differential_v8`: Generate a Wasm module and check that Wasmtime returns
-  the same results as V8.
+  `wasm-spec-interpreter` crate), the `wasmi` interpreter, V8 (through the `v8`
+  crate), or Wasmtime itself run with a different configuration.
 * `instantiate`: Generate a Wasm module and Wasmtime configuration and attempt
   to compile and instantiate with them.
 * `instantiate-many`: Generate many Wasm modules and attempt to compile and