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/table.rs

@@ -4,9 +4,9 @@ use wasmtime::*;
 fn get_none() {
     let store = Store::default();
     let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
-    let table = Table::new(&store, ty, Val::ExternRef(None)).unwrap();
+    let table = Table::new(&store, ty, Val::FuncRef(None)).unwrap();
     match table.get(0) {
-        Some(Val::ExternRef(None)) => {}
+        Some(Val::FuncRef(None)) => {}
         _ => panic!(),
     }
     assert!(table.get(1).is_none());