Use AbstractFunction as a base class to emit leaner code

Avoids mixing in all the specialized apply variants.

We could go further and create an base class for all state machines,
but that would require scala-async to be on the runtime classpath,
which currently isn't a requirement.  For now, I'm keeping it this
way.

Author: retronym

Diff for: src/main/scala/scala/async/internal/AsyncTransform.scala

@@ -49,7 +49,15 @@ trait AsyncTransform {
         List(emptyConstructor, stateVar) ++ resultAndAccessors ++ List(execContextValDef) ++ List(applyDefDefDummyBody, apply0DefDef)
       }
 
-      val tryToUnit = appliedType(definitions.FunctionClass(1), futureSystemOps.tryType[Any], typeOf[Unit])
+      val customParents = futureSystemOps.stateMachineClassParents
+      val tycon = if (customParents.exists(!_.typeSymbol.asClass.isTrait)) {
+        // prefer extending a class to reduce the class file size of the state machine.
+        symbolOf[scala.runtime.AbstractFunction1[Any, Any]]
+      } else {
+        // ... unless a custom future system already extends some class
+        symbolOf[scala.Function1[Any, Any]]
+      }
+      val tryToUnit = appliedType(tycon, futureSystemOps.tryType[Any], typeOf[Unit])
       val template = Template((futureSystemOps.stateMachineClassParents ::: List(tryToUnit, typeOf[() => Unit])).map(TypeTree(_)), emptyValDef, body)
 
       val t = ClassDef(NoMods, name.stateMachineT, Nil, template)