Rename Pledge to FlowEvent

[mratsim]

Dataflow parallelism was introduced in #94.

The term Pledge was coined as a parallel to Promises in the Async world.

However the term does not sound intuitive even though the concept is actually simple.

Besides, from looking into:

• AsyncDispatch with AsyncEvent: https://nim-lang.org/docs/asyncdispatch.html#AsyncEvent
• Chronos with AsyncEvent: https://github.com/status-im/nim-chronos/blob/2518a416/chronos/asyncsync.nim#L27-L38
• Cuda with cudaEvent_t: #133
• OpenCL with cl_event: #134

Event is a commonly accepted term for what the Pledge is doing.

Hence we should rename Pledge to FlowEvent

In parallel, spawnDelayed should be renamed spawnOnEvent. And the dependsOn clause in parallelFor should be renamed waitForEvent to ease grep-ing for event-based / dataflow-based constructs.