new move analyser2 (#20471)

* produce better code for closure environment creation
* new 'first write' analysis; 
* scope based move analyser
* code cleanup

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

tests/destructor/t16607.nim

@@ -15,8 +15,7 @@ proc initO(): O =
   O(initialized: true)
 
 proc pair(): tuple[a, b: O] =
-  result.a = initO()
-  result.b = initO()
+  result = (a: initO(), b: initO())
 
 proc main() =
   discard pair()