scoped memory management (#14790)

* fixes the regressions
* closes #13936
* scope based memory management implemented
* enabled tcontrolflow.nim test case
* final cleanups

tests/casestmt/t12785.nim

@@ -3,6 +3,8 @@ discard """
   output: '''copied
 copied
 2
+destroyed
+destroyed
 copied
 copied
 2
@@ -37,9 +39,9 @@ proc test(a: range[0..1], arg: ObjWithDestructor) =
     inc iteration
 
     case a
-    of 0:  
-        assert false
-    of 1:      
+    of 0:
+      assert false
+    of 1:
       echo b
       if iteration == 2:
         break