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_wasmoved_destroy_pairs.nim

@@ -0,0 +1,92 @@
+discard """
+  output: ''''''
+  cmd: '''nim c --gc:arc --expandArc:main --expandArc:tfor --hint:Performance:off $file'''
+  nimout: '''--expandArc: main
+
+var
+  a
+  b
+  x
+x = f()
+if cond:
+  add(a):
+    let blitTmp = x
+    blitTmp
+else:
+  add(b):
+    let blitTmp_1 = x
+    blitTmp_1
+`=destroy`(b)
+`=destroy`(a)
+-- end of expandArc ------------------------
+--expandArc: tfor
+
+var
+  a
+  b
+  x
+try:
+  x = f()
+  block :tmp:
+    var i
+    var i_1 = 0
+    block :tmp_1:
+      while i_1 < 4:
+        var :tmpD
+        i = i_1
+        if i == 2:
+          return
+        add(a):
+          wasMoved(:tmpD)
+          `=`(:tmpD, x)
+          :tmpD
+        inc i_1, 1
+  if cond:
+    add(a):
+      let blitTmp = x
+      wasMoved(x)
+      blitTmp
+  else:
+    add(b):
+      let blitTmp_1 = x
+      wasMoved(x)
+      blitTmp_1
+finally:
+  `=destroy`(x)
+  `=destroy_1`(b)
+  `=destroy_1`(a)
+-- end of expandArc ------------------------'''
+"""
+
+proc f(): seq[int] =
+  @[1, 2, 3]
+
+proc main(cond: bool) =
+  var a, b: seq[seq[int]]
+  var x = f()
+  if cond:
+    a.add x
+  else:
+    b.add x
+
+# all paths move 'x' so no wasMoved(x); destroy(x) pair should be left in the
+# AST.
+
+main(false)
+
+
+proc tfor(cond: bool) =
+  var a, b: seq[seq[int]]
+
+  var x = f()
+
+  for i in 0 ..< 4:
+    if i == 2: return
+    a.add x
+
+  if cond:
+    a.add x
+  else:
+    b.add x
+
+tfor(false)