Implement LetF shrinking and start AppF shrinking

compiler/src/l3/CPSOptimizer.scala

@@ -55,7 +55,32 @@ abstract class CPSOptimizer[T <: CPSTreeModule { type Name = Symbol }]
   private def shrink(tree: Tree): Tree =
     shrink(tree, State(census(tree)))
 
-  private def shrink(tree: Tree, s: State): Tree = ???
+  private def shrink(tree: Tree, s: State): Tree = tree match {
+    case LetF(funs, body) =>
+      val undeadFuns = funs.filter(fun => !s.dead(fun.name))
+      val undeadShrunkFuns = undeadFuns.map { fun =>
+        Fun(fun.name, fun.retC, fun.args, shrink(fun.body, s))}
+      val (funsAppliedOnce, restFuns) = undeadShrunkFuns
+        .partition(fun => s.appliedOnce(fun.name))
+
+      val newState = s.withFuns(undeadShrunkFuns)
+      val newBody = shrink(body, newState)
+      if (restFuns.isEmpty) {
+        newBody
+      } else {
+        LetF(restFuns, newBody)
+      }
+    
+    case AppF(fun, retC, args) =>
+      fun match {
+        case AtomN(n) if s.fEnv.contains(n) && s.appliedOnce(n) =>
+          // Inline
+          val fun = s.fEnv.get(n)
+          ???
+        case _ => tree
+      }
+
+  }
 
   // (Non-shrinking) inlining