New settings api

[solnic]

This replaces class-level config values with a new settings API powered by dry-configurable. All components have explicit settings defined now and there's a backward-compatibility shim added to rom/compat which makes sure that old-way of configuring classes through class attributes still works.

Here's how the default Relation settings look like:

    setting :auto_map, default: true
    setting :auto_struct, default: false
    setting :struct_namespace, default: ROM::Struct
    setting :wrap_class, default: Relation::Wrap

    setting :component do
      setting :id, default: :relation
      setting :dataset
      setting :adapter
      setting :gateway, default: :default
      setting :inflector, default: Inflector
    end

    setting :schema do
      setting :constant, default: Schema
      setting :dsl_class, default: Schema::DSL
      setting :attr_class, default: Attribute
      setting :inferrer, default: Schema::DEFAULT_INFERRER
    end

There's actually a very strict convention behind this grouping:

• top-level settings are used by a component instance, ie in this case relation objects rely on things like auto_struct at runtime to know how mappers should be inferred
• settings nested under component root key are used by configuration when establishing connections to gateways, figuring out which plugins should be enabled but first-and-foremost - IF a given component should become registered and what its :id should be. ie with component.id set to ":users" a relation will be available as rom.relations[:users] but also rom["relations.users"]
• settings nested under a key that corresponds to a component type, schema in this example, are defaults that will be used when a given component defines its subcomponents. ie a relation can define its schema, so whatever you set here, will end up as defaults for that schema

On top of this, both class definitions and DSL usage is expected to work consistently when it comes to handling configuration settings and DSL options are now automatically translated to component configs.

For example, defining a component class vs defining a component via DSL ends up with the exact same config:

irb(main):017:1* class Users < ROM::Relation[:memory]
irb(main):018:0> end
irb(main):019:0> config = ROM::Configuration.new(:memory)
irb(main):020:0> config.relation(:users)
irb(main):021:0> config.components.get(:relations, id: :users).constant
=> ROM::Relations::Users
irb(main):022:0> config.components.get(:relations, id: :users).constant.config.component
=> #<Dry::Configurable::Config values={:id=>:users, :dataset=>:users, :adapter=>:memory, :gateway=>:default, :inflector=>#<Dry::Inflector>}>
irb(main):023:0> Users.config.component
=> #<Dry::Configurable::Config values={:id=>:users, :dataset=>:users, :adapter=>:memory, :gateway=>:default, :inflector=>#<Dry::Inflector>}>