Make Task a Monadic type

[FreeApophis]

We avoided to make Task a monad so far:

• I regularly miss Sequence on Task<Option> Which would avoid await-hell in quite some places.
• SelectMany with Task Types would allow a lot of simplification.
• Select and SelectMany would swallow exceptions but I think I would be fine with it, because the Monadic interface cannot cause an exception, so the stacktrace only hides the location via indirection.

I suppose there are more downsides but I cannot really evaluate them without some hands-on experience.