Improve waitgroups

[konsumlamm]

• fix some bugs:
  • assert delta >= 0 in enter by making the argument type Natural (Go also supports negative deltas, but those would require special handling)
  • leave does nothing if no tasks have entered (it could also throw an exception, like Go does)
  • only notify when runningTasks is 0 (not a bug fix, but an optimization)
  • use broadcast instead of signal in leave (this allows multiple threads to wait, there is a test for this)
• add tests
• improve documentation

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What do you think about the behaviour of enter if delta < 0 and leave if runningTasks == 0?

[konsumlamm]

#42 should fix the CI.

[Araq]

only notify when runningTasks is 0 (not a bug fix, but an optimization)

I wanted to do this optimization in Malebolgia but forgot. :-)

[narimiran]

#42 should fix the CI.

I also did #43, so we don't test Nim 1.4 anymore.

[konsumlamm]

@Araq what do you think?

[Araq]

It's great, thanks!