Inhibit dynamic method resolution via procCall

Dynamic method resolution can be inhibited via the builtin system.procCall. This is somewhat comparable to the super keyword that traditional OOP languages offer.

type
  Thing = ref object of RootObj
  Unit = ref object of Thing
    x: int
method m(a: Thing) {.base.} =
  echo "base"
method m(a: Unit) =
  # Call the base method:
  procCall m(Thing(a))
  echo "1"