Add an alternative to `Widget::set` that returns a `Result<(), WouldCycle>`

[mitchmindtree]

At the moment, there is no way for a user to handle a cycle created at run-time.

Most of the time this is OK, as users will often instantiate widgets by hand in a natural tree/dag formation. However, for more dynamically controlled widget instantiation, a user might want to handle the case where a cycle would be created.

It would be nice to have a method that offered this, though i'm unsure of a name... .set_result? .set_and_handle_cycle?