added 'reset' magic proc

tests/accept/run/tmacro4.nim

@@ -0,0 +1,19 @@
+discard """
+  output: "after"
+"""
+
+import
+  macros, strutils
+
+macro test_macro*(n: stmt): stmt =
+  result = newNimNode(nnkStmtList)
+  var ass : PNimrodNode = newNimNode(nnkAsgn)
+  add(ass, newIdentNode("str"))
+  add(ass, newStrLitNode("after"))
+  add(result, ass)
+when isMainModule:
+  var str: string = "before"
+  test_macro(str):
+    var i : integer = 123
+  echo str
+