Add benchmark utility that helps address scala#422

* Add just a memo that will tell us how many times we are looking for
the pickler/unpickler of a type `T`.

core/src/main/scala/scala/pickling/generator/PicklingMacros.scala

@@ -1,6 +1,8 @@
 package scala.pickling
 package generator
 
+import scala.collection.mutable
+
 private[pickling] trait PicklingMacros extends Macro with SourceGenerator with TypeAnalysis {
 
   implicit val implContext = c
@@ -49,6 +51,7 @@ private[pickling] trait PicklingMacros extends Macro with SourceGenerator with T
     }
   }
 
+
   def preferExistingImplicits(body: => Tree): Tree = {
 
     import Compat._
@@ -58,11 +61,15 @@ private[pickling] trait PicklingMacros extends Macro with SourceGenerator with T
     if (candidates.isEmpty) return body
     val ourPt = candidates.head.pt
 
+    val key = ourPt.toString
+    Memo.memo += key -> (Memo.memo.getOrElse(key, 0) + 1)
+
     def debug(msg: Any) = {
       val padding = "  " * (candidates.length - 1)
       Console.err.println(padding + msg)
     }
 
+    debug(YELLOW_B + s"We have entered here ${Memo.memo.get(key)}" + RESET)
     debug(MAGENTA_B + "Can we enter " + ourPt + "?" + RESET)
     debug(candidates)
 
@@ -102,3 +109,7 @@ private[pickling] trait PicklingMacros extends Macro with SourceGenerator with T
   }
 
 }
+
+object Memo {
+  val memo = mutable.Map.empty[String, Int]
+}