Support async operations in properties

[AlexeyRaga]

Async support with ergonomics like:

property {
    let! x = Gen.int32 (Range.constant 0 100)
    let! y = async { return x + 1 }  // ensures we can call Async<T> function
    let! z = task { return y + 1 }   // ensures that we can call Task<T> function
    return z = x + 2
 }

var property =
    from x in Gen.Int32(Range.Constant(0, 100)).ForAll()
    from y in Task.FromResult(x + 1) // ensures that we can call Task<T> function
    from z in Task.FromResult(y + 1)
    select z == x + 2;