Add .cast() methods

[thosakwe]

Would love to send in a quick PR for this.

Pseudo-code example:

Tuple3 doSomething() native "_doSomething";

void main() {
  var tuple = doSomething.cast<X, Y, Z>(); // Tuple<X, Y, Z>
  // Do something else...
}

class Tuple3 {
  Tuple3<T, U, V> cast<T, U, V>() {
    return Tuple3(item1 as T, item2 as U, item3 as V);
  }
}

[devoncarew]

Thanks fort the issue! However, with the recent release of Dart 3.0 and its support for Records, we plan to move this package into maintenance mode. That would essentially mean not take any enhancements to it and just maintaining it from the POV of bug fixes / CI updates to keep it working.

Some more detail in the related issue: #47.