Remove Mobility::Interface

[shioyama]

Mobility currently has an interface for accessing certain model-global methods like names which returns the attribute names on the model (for all backends). This is accessible via Post.mobility.

However, with the extensive changes in #225, I'm finding that having both the class query scope Post.i18n and the interface on Post.mobility is getting pretty clumsy. Here I am going to try to move this interface stuff into the Attributes module builder so that class methods are defined on the model as soon as any attributes are defined, without using a memoized instance variable (which requires extra precautions when duping etc.)

The goal here is to have this type of interface for accessors and backends:

class Post < ApplicationRecord
  extend Mobility
  translates :title
end

post = Post.new
post.mobility_backends
#=> {}
post.title = "foo"
post.mobility_backends
#=> {:title=> <memoized instance of backend class>}
Post.mobility_backend_class(:title)
#=> #<Mobility::Backends::ActiveRecord::...>

In the i18n scope, you also get the [] shortcut to access nodes:

Post.i18n[:title]
#=> Arel node for `title` attribute

Although I made querying into a plugin (on the master branch only), I'm going to convert it into a normal module which is included by default when you extend Mobility. Querying in the end is something global that should only enabled on an entire model, or not at all.

[shioyama]

I've gone with backend_node as the name of the method to get the node on the query scope:

Post.i18n.backend_node(:title)
#=> Arel node for title attribute

Anything generic like [] will possibly conflict with other scope methods added by gems, etc.