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/invoke_func_via_table.rs

@@ -23,6 +23,7 @@ fn test_invoke_func_via_table() -> Result<()> {
         .unwrap()
         .funcref()
         .unwrap()
+        .unwrap()
         .clone();
     let result = f.call(&[]).unwrap();
     assert_eq!(result[0].unwrap_i64(), 42);