wasmtime: Add support for func.ref and table.grow with funcrefs

`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`.

This should be more efficient than using a `VMExternRef` that points at a
`VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic
allocation, and some reference counting.

Note that the null function reference is *NOT* a null pointer; it is a
`VMCallerCheckedAnyfunc` that has a null `func_ptr` member.

Part of #929

tests/all/main.rs

@@ -21,4 +21,26 @@ mod wast;
 
 // TODO(#1886): Cranelift only supports reference types on x64.
 #[cfg(target_arch = "x86_64")]
+mod funcref;
+#[cfg(target_arch = "x86_64")]
 mod gc;
+
+/// A helper to compile a module in a new store with reference types enabled.
+#[cfg(target_arch = "x86_64")]
+pub(crate) fn ref_types_module(
+    source: &str,
+) -> anyhow::Result<(wasmtime::Store, wasmtime::Module)> {
+    use wasmtime::*;
+
+    let _ = env_logger::try_init();
+
+    let mut config = Config::new();
+    config.wasm_reference_types(true);
+
+    let engine = Engine::new(&config);
+    let store = Store::new(&engine);
+
+    let module = Module::new(&engine, source)?;
+
+    Ok((store, module))
+}