Update Relation to subclass Memory::Relation

Recent changes to `ROM::Memory::Relation` made it possible to use it as
the superclass for adapter relations. This adds schema support among
other things.

lib/rom/csv/relation.rb

@@ -1,4 +1,4 @@
-require 'rom/relation'
+require 'rom/memory'
 
 module ROM
   module CSV
@@ -9,11 +9,9 @@ module CSV
     #   end
     #
     # @api public
-    class Relation < ROM::Relation
+    class Relation < ROM::Memory::Relation
       adapter :csv
 
-      forward :join, :project, :restrict, :order
-
       def count
         dataset.count
       end

spec/integration/gateway_spec.rb

@@ -9,6 +9,12 @@
       configuration.relation(:users) do
         gateway :users
 
+        schema(:users) do
+          attribute :user_id, Types::Strict::Int
+          attribute :name, Types::Strict::String
+          attribute :email, Types::Strict::String
+        end
+
         def by_name(name)
           restrict(name: name)
         end