Implement `then` and `combinePrevious` equivalents for Rx
Implement `then` and `combinePrevious` equivalents for Rx

Following Swift's value-type, non-mutating nomenclature, these have been named `then(observer:)` and `combiningPrevious(startingWith:)` for better readability

chamander/Sampling-Reactive,g-ackt/Sampling-Rx,chamander/Sampling-Reactive,g-ackt/Sampling-Rx