Replace -asMaybes with -materialize/-dematerialize

`-materialize` matches Rx, and, unlike `-asMaybes`, brings the `completed` event into the stream of values. This means that all events can be bound, mapped, filtered, etc. following materialization, and improves conversion to and from `RACSequence`.

It does _not_, however, resubscribe to the signal like `-asMaybes` did. This is trivial to do using `-repeat` and/or `-retry` instead.