Introduce an Attribute object to handle the type casting dance

There's a lot more that can be moved to these, but this felt like a good
place to introduce the object. Plans are:

- Remove all knowledge of type casting from the columns, beyond a
  reference to the cast_type
- Move type_cast_for_database to these objects
- Potentially make them mutable, introduce a state machine, and have
  dirty checking handled here as well
- Move `attribute`, `decorate_attribute`, and anything else that
  modifies types to mess with this object, not the columns hash
- Introduce a collection object to manage these, reduce allocations, and
  not require serializing the types

1 files changed, 3 insertions, 4 deletions

diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index dc8f25b0e7..6d1f72eb7e 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1488,15 +1488,14 @@ class BasicsTest < ActiveRecord::TestCase
     attrs = topic.attributes.dup
     attrs.delete 'id'
 
-    typecast = Class.new {
-      def type_cast_from_database value
+    typecast = Class.new(ActiveRecord::Type::Value) {
+      def type_cast value
         "t.lo"
       end
     }
 
     types = { 'author_name' => typecast.new }
-    topic = Topic.allocate.init_with 'raw_attributes' => attrs,
-                                     'column_types' => types
+    topic = Topic.instantiate(attrs, types)
 
     assert_equal 't.lo', topic.author_name
   end