An optimizer for ARC (#14962)

* WIP: an optimizer for ARC
* do not optimize away destructors in 'finally' if unstructured control flow is involved
* optimized the optimizer
* minor code cleanup
* first steps to .cursor inference
* cursor inference: big steps to a working solution
* baby steps
* better .cursor inference
* new feature: expandArc for easy inspection of the AST after ARC transformations
* added topt_cursor test
* adapt tests
* cleanups, make tests green
* optimize common traversal patterns
* moved test case
* fixes .cursor inference so that npeg compiles once again
* cursor inference: more bugfixes

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/destructor/tuse_result_prevents_sinks.nim

@@ -17,15 +17,20 @@ proc `=sink`(self: var Foo; other: Foo) =
 
 proc `=destroy`(self: var Foo) = discard
 
+template preventCursorInference(x) =
+  let p = unsafeAddr(x)
+
 proc test(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
   return result
 
 proc testB(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
 
 discard test()