continuous operators should skip work on duplicates

[leonoel]

When a continuous flow is sampled, if the current value is = to the previous one, it is not a change and nothing meaningful should happen.

• latest : the combinator function should not be called if no input has actually changed, the previous result is returned instead.
• ?< : the switch should not happen if the parent has not changed, the current child branch is kept alive and reused. cp should defer detaching until sampling and return the previous result if no branch has actually changed.

[leonoel]

Released in b.27