Async error tracking

[nitely]

Somewhat inspired in chronos, but kinda the opposite.

Goal:

• Make async raises work exactly the same as non-async code.
• Callbacks must not raise for this to be sane. Albeit, I think code that passes callbacks that raise is broken anyway. It's possible to make this backward compatible, I think. Just deprecate the proc that takes a cb without raises: [], and cast it to untrack raises.
• Provide a way to pass error typed Futures around. There must be a way to get the error typed future of an async call.
• Backward compatible

Todo:

• tests
• fix for non-void Future returns, so it returns FutureUntracked
• make all, and, or not raise Exception
• make fail for tracked exceptions only allow listed errors
• fix introduced XXX
• This issue Generic inheritance matching gives type mismatch when object has members nim-lang/Nim#22936 needs to be fixed to be able to make FutureUntracked an object instead of an alias. But it seems alias works fine so far.
• check all nimble packages using async compile

PR upstream once it looks like this is even possible.