arc optimizations (#13325)

* scope based destructors
* handle 'or' and 'and' expressions properly, see the new test arc/tcontrolflow.nim
* make this branch mergable, logic is disabled for now

lib/system.nim

@@ -1954,14 +1954,7 @@ template newException*(exceptn: typedesc, message: string;
                        parentException: ref Exception = nil): untyped =
   ## Creates an exception object of type ``exceptn`` and sets its ``msg`` field
   ## to `message`. Returns the new exception object.
-  when declared(owned):
-    var e: owned(ref exceptn)
-  else:
-    var e: ref exceptn
-  new(e)
-  e.msg = message
-  e.parent = parentException
-  e
+  (ref exceptn)(msg: message, parent: parentException)
 
 when hostOS == "standalone" and defined(nogc):
   proc nimToCStringConv(s: NimString): cstring {.compilerproc, inline.} =