Fix Command<T>::perform to return a Command<T>

This seems like clearly the correct thing to do here. If the type bound
on `Command` isn't specified, it makes no difference, since the generic
is inferred in a way that works with either definition. But this is
important if `Command<T>` is aliased with a concrete type.

runtime/src/command.rs

@@ -40,9 +40,9 @@ impl<T> Command<T> {
 
  /// Creates a [`Command`] that performs the action of the given future.
  pub fn perform<A>(
- future: impl Future<Output = T> + 'static + MaybeSend,
- f: impl FnOnce(T) -> A + 'static + MaybeSend,
- ) -> Command<A> {
+ future: impl Future<Output = A> + 'static + MaybeSend,
+ f: impl FnOnce(A) -> T + 'static + MaybeSend,
+ ) -> Command<T> {
  use iced_futures::futures::FutureExt;
 
  Command::single(Action::Future(Box::pin(future.map(f))))