Introduces Hanami::Relation

[mereghost]

:warning: Achtung: This is a true draft PR. The code is messy. The commit history is changing all the time and broken tests are a thing. :warning:

This PR is the first step towards bringing model closer to rom.

In this we introduce the Relation abstraction in the same terms as rom does. This simplifies our code, boot process and exposes more of ROM to the users.

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Most important changes

Modules instead of suffixes

Repositories, Entities and Relations are expected to be under a certain directory structure that should reflect a module structure. This follows the same rules for ROM auto-registration feature

Bookshelf::Relations::Books   #=> should exist under bookshelf/relations/books.rb

• [x] Relations
• [x] Entities
• [ ] Repositories

Hanami::Relation

This PR introduces Hanami::Relation which maps directly to a ROM::Relation and offers the exact same API (being only an alias to it).

class ProjectName::Relations::Users < Hanami::Relation[:sql]
  schema(infer: true) do
    associations do
      has_many :books
    end
  end
end

Hanami::Repository

Hanami::Repository now takes the name of its root relation. Overall functionality is the same.

# This is moving to module namespacing soon
class BookRepository < Hanami::Repository[:books]
end

Hanami::Entity

Entity becomes a ROM::Struct

TODO

• [ ] Update YARD docs
• [ ] Remove unnecessary tests due to the new approach
• [ ] Find a way to silence Repository method override (create, delete, update) (or remove if not needed)
• [ ] Setup an easy way to instantiate an Entity (ROM)
• [ ] Workout why some queries return ROM::Struct and others Hanami::Entity forcing both Relation and Repository to have the struct_namespace macro on them
• [ ] Work out some unexpected behaviour:

class Books < ROM::Relation[:sql]
  schema(infer: true) do
    associations { belongs_to :author }
  end
end

class Authors < ROM::Relation[:sql]
  schema(infer: true) do
    associations { has_many :books }
  end
end

class Comments < ROM::Relation[:sql]
  schema(infer: true) do
   associations { belongs_to :user, as: :author }
  end
end

class Users < ROM::Relation[:sql]
  schema(infer: true) do
    associations { has_many :comments }
  end
end

class Repositories::Comment < ROM::Repository[:comments]
  def with_author
    root.combine(:author).to_a
  end
end

Repositories::Comment.new.with_author.first.author
#=> <ROM::Struct::Author id=50 name="Bob from the Upside Down" email=nil age=19 comments_count=0 active=true created_at=2020-07-17 14:20:24 UTC updated_at=2020-07-17 14:20:24 UTC>

In the case above, the returned struct is in fact a User, but ROM renamed it Author as the association is aliased (and this is kinda neat). Why this can become an issue? In the above example the project has an Author entity and relation, so this can get a little confusing.