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

build.rs

@@ -215,7 +215,9 @@ fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
 
             ("reference_types", "table_copy_on_imported_tables")
             | ("reference_types", "externref_id_function")
-            | ("reference_types", "table_size") => {
+            | ("reference_types", "table_size")
+            | ("reference_types", "simple_ref_is_null")
+            | ("reference_types", "table_grow_with_funcref") => {
                 // TODO(#1886): Ignore if this isn't x64, because Cranelift only
                 // supports reference types on x64.
                 return env::var("CARGO_CFG_TARGET_ARCH").unwrap() != "x86_64";