module Gorillib::Model::PositionalFields

give each field the next position in order

@example

class Foo
  include Gorillib::Model
  field :bob,    String               # not positional
  field :zoe,    String, position: 0  # positional 0 (explicit)
end
class Subby < Foo
  include Gorillib::Model::PositionalFields
  field :wun, String                  # positional 1
end
Foo.field    :nope, String            # not positional
Subby.field  :toofer, String          # positional 2

@note: make sure you’re keeping positionals straight in super classes, or in anything added after this.