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

tests/destructor/tasync_prototype.nim

@@ -32,7 +32,12 @@ proc serve(server: PAsyncHttpServer): PFutureBase =
       yield acceptAddrFut
       var fut = acceptAddrFut.value
 
+      # with the new scope based destruction, this cannot
+      # possibly work:
       var f {.cursor.} = processClient()
+      # It also seems to be the wrong way how to avoid the
+      # cycle. The cycle is caused by capturing the 'env'
+      # part from 'env.f'.
       when true:
         f.callback =
           proc () =

tests/destructor/tdestructor.nim

@@ -33,7 +33,7 @@ type
     p: pointer
 
 proc `=destroy`(o: var TMyObj) =
-  if o.p != nil: 
+  if o.p != nil:
     dealloc o.p
     o.p = nil
     echo "myobj destroyed"

tests/destructor/tmisc_destructors.nim

@@ -22,7 +22,7 @@ proc `=`(dest: var Foo, src: Foo) =
   assign_counter.inc
 
 proc test(): auto =
-  var a,b : Foo
+  var a, b: Foo
   return (a, b, Foo(boo: 5))
 
 var (a, b, _) = test()

tests/destructor/tv2_raise.nim

@@ -2,7 +2,7 @@ discard """
   valgrind: true
   cmd: '''nim c -d:nimAllocStats --newruntime $file'''
   output: '''OK 3
-(allocCount: 8, deallocCount: 3)'''
+(allocCount: 8, deallocCount: 5)'''
 """
 
 import strutils, math