Allow use of `Task`, as opposed to `Task<'T>` in `do!` statements

[abelbraaksma]

Currently, this is not possible:

taskSeq {
   do! Task.Delay 220  // fails, because this returns a Task, not a Task<unit>
}

Add necessary overloads for allowing this, similarly to how the task CE works.

Of note is that allowing Task can lead to unexpected behavior, as Task<'T> inherits from Task. However, we should strive to have parity with task in F#. to avoid surprises for users. Just like with normal task, we would still disallow do! task { return 10 }. This should be OK, as the F# compiler does not (yet) allow that kind of conversion.

[abelbraaksma]

Solved in #109, will be part of v0.3.0.