More Reflex.Time and related functions

[lspitzner]

See https://gist.github.com/lspitzner/18e679006765b308e9dbc41b06e86bbb

• gateGather: an accumulating gate
• maskCalmE: behaves like throttle, but not controlled by time, but by input dynamic
• conditionalBuffer: a buffer that can conditionally retain one input event value
• fanBy: (eventsWhenFalse, eventWhenTrue) = fanBy pred inputE
• fanOn: (eventsWhenFalse, eventWhenTrue) = fanOn boolBeh inputE
• triggerDelay: behaves like delay, but not controlled by time, but my a second input event
• executeAsync1Calm: perform an event asynchronously, but do not run more than one at a time
• executeAsync1CalmWith: with custom forker function
• gateThrottle: like throttle but without any accumulation. better for debouncing bursts.

All names open for discussion.

Only gateThrottle actually uses time, so I doubt that Reflex.Time is the right module, but not sure where else these would belong.

[lspitzner]

I have added conditionalBuffer because it was necessary to fix the semantics of executeAsync1Calm(With). Preventing logic loops when inputs get connected to outputs is tricky.. hope things are fixed now.

[lspitzner]

To be clear, I am ready to turn this into a PR, presuming that there is interest in having these included and that we decide on which module to put these in.