Mixin statement

A symbol can be forced to be open by a mixin declaration:

  1. proc create*[T](): ref T =
  2.   # there is no overloaded 'init' here, so we need to state that it's an
  3.   # open symbol explicitly:
  4.   mixin init
  5.   new result
  6.   init result

mixin statements only make sense in templates and generics.