pairwise method
Groups the items emitted by an Observable into a record type that represent the latest pair of items emitted by the source Observable.
For example [1, 2, 3, 4].toObservable().pairwise() yields (1, 2),
(2, 3), and (3, 4).
Implementation
Observable<(T, T)> pairwise() => PairwiseObservable<T>(this);