Extract mapper-related features into rom-mapper

[wmaciejak]

Issue discovered while upgrading ROM to version 2.0.0.

rom (= 2.0.0) was resolved to 2.0.0, which depends on
      rom-mapper (~> 0.4.0)

Other, related issue - when you will try use mapper on combined records, then you will see warnings like this

[1] pry(#<UserRepository>)> users.combine(one: { settings: [settings.for_org(org), id: :user_id] }).as(:entity).to_a
/Users/user/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/transproc-0.4.0/lib/transproc/hash.rb:246: warning: wrong element type Hash at 0 (expected array)
/Users/user/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/transproc-0.4.0/lib/transproc/hash.rb:246: warning: ignoring wrong elements is deprecated, remove them explicitly
/Users/user/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/transproc-0.4.0/lib/transproc/hash.rb:246: warning: this causes ArgumentError in the next release
=> [{:settings=>{:active=>false}}, {:settings=>{:active=>false}}]

Mapper are very simple

module UserMappers
  class Entity < ROM::Mapper
    relation :users
    register_as :entity
    reject_keys true

    attribute :id
    attribute :code
    attribute :name
    attribute :settings do |settings|
      settings ? settings[:value].deep_symbolize_keys : { active: false }
    end
  end
end

[solnic]

Done in master