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/use_after_drop.rs

@@ -11,10 +11,10 @@ fn use_func_after_drop() -> Result<()> {
             assert_eq!(closed_over_data, "abcd");
         });
         let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
-        table = Table::new(&store, ty, Val::ExternRef(None))?;
+        table = Table::new(&store, ty, Val::FuncRef(None))?;
         table.set(0, func.into())?;
     }
-    let func = table.get(0).unwrap().funcref().unwrap().clone();
+    let func = table.get(0).unwrap().funcref().unwrap().unwrap().clone();
     let func = func.get0::<()>()?;
     func()?;
     Ok(())