support has_many in YAML sources

class Plans::IsoDistro < ActiveYaml::Base
  include ActiveHash::Associations
  has_many :iso_images, class_name: 'Plans::IsoImage'
end

class Plans::IsoImage < ActiveHash::Base
  include ActiveHash::Associations
  belongs_to :iso_distro, class_name: 'Plans::IsoDistro'
end

- id: 1
  code: arch
  name: Arch Linux
  iso_images:
    - path: archlinux.iso
      bit: 64
    - path: ubuntu.iso
      bit: 32

@isos.first
=> #<Plans::IsoDistro:0x007f75b8962348 @attributes={:id=>1, :code=>"arch", :name=>"Arch Linux", :iso_images=>[{"path"=>"archlinux.iso", "bit"=>64}, {"path"=>"ubuntu.iso", "bit"=>32}]}>
@isos.first.iso_images
=> []

This could understand nested structures and create IsoImage objects too, along with an appropriate relation.

Start by tracing how YAML sources are loaded and how the existing has_many and belongs_to associations resolve records; the issue provides the Plans::IsoDistro and Plans::IsoImage entry points and nested YAML example. Done means nested iso_images produce IsoImage objects and @isos.first.iso_images returns the appropriate relation, with behavior covered by tests.