• Adds from operator on "SharedSequence"
• Adds merge operator on "Completable"
• Adds using operator on "PrimitiveSequence"
• Adds concatMap operator.
• Adds share(replay:scope:) operator.
• Adds multicast(makeSubject:) operator.
• Adds UIButton.image(for:) extension.
• Adds UIButton.backgroundImage(for:) extension.
• fixes typos

Anomalies

• Improves reentrancy and synchronization checks.
• Issues with share() and shareReplay(_:). #1111
• .share() inconsistent in behavior. #1242
• Fixes issues with Driver sometimes sending initial element async. #1253