Extend try/finally block to work with awaitable finalizers

[Horusiath]

This is my first PR, so some help will be needed eg. how do you test this?

[NinoFloris]

Ha, this grew from a little research project to something that's seriously used a bit too quickly.
Robert (from TaskBuilder.fs) has given permission to use their tests though!

Does this PR actually allow something like the following to compile?

let foo = task {
    try 
      return ()
    finally
      let! x = Task.Yield()
}

[Horusiath]

It looks like it doesn't work that way - F# compiler lags behind.

[NinoFloris]

Nothing to merge here then :/