Disable reference types in v8 fuzzing (#4387)

OSS-fuzz long-ago discovered https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=45662
which we currently believe to be a bug in v8. I originally thought it
was going to be fixed with https://bugs.chromium.org/p/v8/issues/detail?id=12722
but that no longer appears to be the case now that the `v8` crate has
caught up and it still isn't fixed. Personally I've sort of lost an
appetite for continuing to debug these issues so I figure it's best to
just disable reference types with v8 for now and exercise the rest of
the engine, e.g. simd.

fuzz/fuzz_targets/differential_v8.rs

@@ -19,7 +19,16 @@ fn run(data: &[u8]) -> Result<()> {
     // Enable features that v8 has implemented
     config.module_config.config.simd_enabled = u.arbitrary()?;
     config.module_config.config.bulk_memory_enabled = u.arbitrary()?;
-    config.module_config.config.reference_types_enabled = u.arbitrary()?;
+
+    // FIXME: reference types are disabled for now as we seemingly keep finding
+    // a segfault in v8. This is found relatively quickly locally and keeps
+    // getting found by oss-fuzz and currently we don't think that there's
+    // really much we can do about it. For the time being disable reference
+    // types entirely. An example bug is
+    // https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=45662
+    //
+    // config.module_config.config.reference_types_enabled = u.arbitrary()?;
+
     // FIXME: to enable fuzzing with the threads proposal, see
     // https://github.com/bytecodealliance/wasmtime/issues/4268.
     // config.module_config.config.threads_enabled = u.arbitrary()?;