This replaces the single, `Try`-dependent `completeProm` method with:

    def completeProm[A: WeakTypeTag](
      prom: Expr[Prom[A]], value: Expr[A]): Expr[Unit]

    def completePromWithExceptionTopLevel[A: WeakTypeTag](
      prom: Expr[Prom[A]], exception: Expr[Throwable]): Expr[Unit]

    def completePromWithFailedResult[A: WeakTypeTag](
      prom: Expr[Prom[A]], resultName: TermName): Expr[Unit]

Adds the following methods to `FutureSystem#Ops`:

    def isFailedResult(name: TermName): Expr[Boolean]
    def resultValue(name: TermName, resultType: Type): Tree

Introduces `TryBasedFutureSystem` trait.

Adds `Result[A]` type member to `FutureSystem` and
`def resultType[A]` to `FutureSystem#Ops`.

Removes method `TransformUtils.applied`.

- async { } block can now generate a subclass of an existing trait
- future system can directly return the state machine instance
  from async, without spawning an actual future
- adds iterator-based async implementation with future system