Refactor serialized types to be partially defined as custom properties

Many of the methods defined in `AttributeMethods::Serialization` can be
refactored onto this type as well, but this is a reasonable small step.
Removes the `Type` class, and the need for `decorate_columns` to handle
serialized types.
Merge pull request #15398 from sgrif/sg-serialized-types

Refactor serialized types to be partially defined as custom properties
Fix broken link in Rails 3.0 release_notes
Merge pull request #15411 from nathany/patch-1

Fix broken link in Rails 3.0 release_notes [ci skip]
Add missing period from 4b802bc [ci skip]
Move `type_cast_for_write` behavior over to the serialized type object
Merge pull request #15412 from sgrif/sg-serialized-type-cast-for-write

Move `type_cast_for_write` behavior over to the serialized type object
Don't reference comments that do not exist
Merge pull request #15413 from sgrif/sg-i-wish-i-could

Don't reference comments that do not exist
Generated engines don't include bin/rake, fixes #15409 [ci skip]