Performance improvements to the macro

[retronym]

Avoid boxing

Don't box state IDs in collections.

Avoid needless string to name conversion

• Introduce a per-Global store of names
• Avoid double freshening names
• Use $async$ in all names to avoid clashes with, e.g. lambda lifted methods.

Rework dead state elimination to happen as part of expr builder

If we're in the expr position of a block, the nested state
generation can reuse the successor state.

Also, output .dot graph of state machine in verbose mode.

Sample: https://gist.github.com/88225478b11c118609b9348d61e13630

View with a local Graphviz install or http://graphviz.it/#/gallery/unix.gv

Sample generated from late expansion of:

val result = run(
  """
    import scala.async.run.late.{autoawait,lateasync}
    case class FixedFoo(foo: Int)
    class Foobar(val foo: Int, val bar: Double) {
      def guard: Boolean = true
      @autoawait @lateasync def getValue = 4.2
      @autoawait @lateasync def func(f: Any) = {
        ("": Any) match {
          case (x1, y1) if guard => x1.toString; y1.toString
          case (x2, y2) if guard => x2.toString; y2.toString
          case (x3, y3) if guard => x3.toString; y3.toString
          case (x4, y4) =>
             getValue; x4.toString; y4.toString
        }
      }
    }
    object Test {
      @lateasync def test() = new Foobar(0, 0).func(4)
    }
    """)

[retronym]

TODO:

• Where is State 19 in the demo? I thought the state IDs were compacted by construction.
• Add an API for the a plugin phase that use the macro implementation to selectively generate the dot
• Print the code snippets with the rewritten states IDs, we shouldn't see stateMachine$async.this.state$async = -11206
• Print the input source code in a text box at the top of the diagram.

[retronym]

I've improved the .dot output.

• The compacted state IDs are shown in the code snippets (the same ones that appear in the generated code)
• Code is left justified, and the enclosing block is elided
• Two nodes are shown for async boundaries (that's why state 19 appeared to be missing above). The edge between them is styled differently to show that it is async.

Updated sample: https://gist.github.com/bbd3312404a9259e4c0f0135bf5d34e3