Async exception tracking

[nitely]

Somewhat inspired in chronos.

Goal:

• Enable Exception Tracking for async. Using the native Nim exception tracking.
• Enable passing error typed Futures around.
• No runtime penalties.
• Backward compatible.

How:

• Await retrieves the raises list for the future. If the raises list cannot be resolved, then error gets inferred as Exception (the current behavior).
• toFutureEx macro takes an async function call, and it returns the error typed future FutureEx[T, E] for passing it around.
• Callbacks must not raise for this to work. 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.

Todo:

• tests
• implement all, and, or FutureEx versions
• fail proc for tracked exceptions; can it be done so it only accepts the type exceptions?
• fix all XXX
• check all nimble packages using async compile