Support multi-assign in function calls

It would be useful to support multi-assign in function parameters. For example int add([int a, int b], int c) { return a + b + c; }, to be called add(({ 1, 2 }), 3). This would be particularly useful in asynchronous code using promises and ->then(), as in

return fetch_location()
  ->then(lambda(mapping location_info) {
           return Concurrent.all(fetch_weather(location_info->lat, location_info->long), fetch_time(location_info->timezone));
         }
  ->then(lambda([mapping weather, string localtime]) {
           return sprintf("It is %d degrees, %s at %s.", weather->temp, weather->summary, localtime));
         });