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/arc/topt_refcursors.nim

@@ -0,0 +1,39 @@
+discard """
+  output: ''''''
+  cmd: '''nim c --gc:arc --expandArc:traverse --hint:Performance:off $file'''
+  nimout: '''--expandArc: traverse
+
+var it = root
+block :tmp:
+  while (
+    not (it == nil)):
+    echo [it.s]
+    it = it.ri
+var jt = root
+block :tmp_1:
+  while (
+    not (jt == nil)):
+    let ri_1 = jt.ri
+    echo [jt.s]
+    jt = ri_1
+-- end of expandArc ------------------------'''
+"""
+
+type
+  Node = ref object
+    le, ri: Node
+    s: string
+
+proc traverse(root: Node) =
+  var it = root
+  while it != nil:
+    echo it.s
+    it = it.ri
+
+  var jt = root
+  while jt != nil:
+    let ri = jt.ri
+    echo jt.s
+    jt = ri
+
+traverse(nil)