Add lua_xpush to 5.1-5.4

src/ffi/lua53/lua.rs

@@ -434,6 +434,12 @@ pub unsafe fn lua_replace(L: *mut lua_State, idx: c_int) {
     lua_pop(L, 1)
 }
 
+#[inline(always)]
+pub unsafe fn lua_xpush(from: *mut lua_State, to: *mut lua_State, idx: c_int) {
+    lua_pushvalue(from, idx);
+    lua_xmove(from, to, 1);
+}
+
 //
 // Debug API
 //