[indystructural] Prototype structural calls using invokedynamic

See also: https://github.com/retronym/indy-structural

```
// sandbox/structural.scala
object Test {
  def main(args: Array[String]): Unit = {
    println(duckduck(new C))
    println(duckduck(new D))
  }

  def duckduck(a: { def quack(a: String): String }): String = {
    a.quack("A")
  }

}

class C {
  def quack(a: String) = "QUACK! " + a
}

class D {
  def quack(a: String) = "QUICK! " + a
}
```

```
qscalac -target:jvm-1.8 -Ybackend:GenBCode sandbox/structural.scala && qscala Test && javap -v 'Test$' | cat -v
warning: there was one feature warning; re-run with -feature for details
one warning found

QUACK! A
QUICK! A

Classfile /Users/jason/code/scala2/Test$.class
{
  public static final Test$ MODULE$;
    descriptor: LTest$;
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL

  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=4, locals=2, args_size=2
         0: getstatic     #19                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
         3: aload_0
         4: new           #21                 // class C
         7: dup
         8: invokespecial #22                 // Method C."<init>":()V
        11: invokevirtual #26                 // Method duckduck:(Ljava/lang/Object;)Ljava/lang/String;
        14: invokevirtual #30                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
        17: getstatic     #19                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
        20: aload_0
        21: new           #32                 // class D
        24: dup
        25: invokespecial #33                 // Method D."<init>":()V
        28: invokevirtual #26                 // Method duckduck:(Ljava/lang/Object;)Ljava/lang/String;
        31: invokevirtual #30                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
        34: return

  public java.lang.String duckduck(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_1
         1: ldc           #38                 // String A
         3: invokedynamic #49,  0             // InvokeDynamic #0:"dyn:callMethod:quack":(Ljava/lang/Object;Ljava/lang/String;)Ljava/lang/String;
         8: checkcast     #51                 // class java/lang/String
        11: areturn

}
BootstrapMethods:
  0: #45 invokestatic jdk/internal/dynalink/DefaultBootstrapper.bootstrap:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    Method arguments:

```

Author: retronym

src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala

@@ -311,6 +311,17 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
         case app : Apply =>
           generatedType = genApply(app, expectedType)
 
+        case app @ ApplyDynamic(qual, args) if settings.target == "jvm.1.8" =>
+            genLoad(qual)
+            val symbol = app.symbol
+            val paramTypes = symbol.info.paramTypes map toTypeKind
+            genLoadArguments(args, paramTypes)
+            val argAsmTypes = symbol.info.params.map(symInfoTK).map(_.toASMType)
+            val returnAsmType = toTypeKind(symbol.info.resultType).toASMType
+            val invokedType = asm.Type.getMethodType(returnAsmType, toTypeKind(qual.tpe).toASMType +: argAsmTypes: _*)
+            val name = "dyn:callMethod:" + symbol.name
+            mnode.visitInvokeDynamicInsn(name, invokedType.getDescriptor, dynalinkBoostrapHandle)
+
         case ApplyDynamic(qual, args) => sys.error("No invokedynamic support yet.")
 
         case This(qual) =>
@@ -1324,4 +1335,11 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
       definitions.LambdaMetaFactory.fullName('/'), sn.AltMetafactory.toString,
       "(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;")
 
+  // TODO use custom boostrap method in our runtime that employs ChainedCallSite to cache recently used linkage results
+  // TODO use "org.dynalang" % "dynalink" rather than relying on Nashorn internals
+  lazy val dynalinkBoostrapHandle =
+    new asm.Handle(asm.Opcodes.H_INVOKESTATIC,
+      "jdk/internal/dynalink/DefaultBootstrapper", "bootstrap",
+      "(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;")
+
 }

src/compiler/scala/tools/nsc/transform/CleanUp.scala

@@ -446,7 +446,7 @@ abstract class CleanUp extends Statics with Transform with ast.TreeDSL {
        *   refinement, where the refinement defines a parameter based on a
        *   type variable. */
 
-      case tree: ApplyDynamic =>
+      case tree: ApplyDynamic if !(settings.isBCodeActive && settings.target.value == "jvm-1.8") =>
         transformApplyDynamic(tree)
 
       /* Some cleanup transformations add members to templates (classes, traits, etc).