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/gc.rs

@@ -1,21 +1,8 @@
+use super::ref_types_module;
 use std::cell::Cell;
 use std::rc::Rc;
 use wasmtime::*;
 
-fn ref_types_module(source: &str) -> anyhow::Result<(Store, Module)> {
-    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))
-}
-
 #[test]
 fn smoke_test_gc() -> anyhow::Result<()> {
     let (store, module) = ref_types_module(