Get rid of ActiveModel::Configuration, make better use of
ActiveSupport::Concern + class_attribute, etc.

copy the columns hash to the active record instances, typecast using columns looked up on the instance

another value has been specified).

This reverts commit be7d2248.

too.

The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.

So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.

The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.

This reverts commit f6b50463.

Fear not, the roflscale will return when I have a bit more time and
figure out a better way to do it. (In particular, a way that doesn't
break the build.)

Don't prefix the name with attribute_. Avoids a string allocation on
read_attribute, which is a bit faster.

This means we never have to rely on define_method (which is slower and
uses more memory), even when we have attributes containing characters
that are not allowed in standard method names.

(I am mainly changing this because the duplication annoys me, though.)

Also make it actually work.

It slows down all read_attribute accesses to map 'id' to whatever the PK
actually is, inside read_attribute. So instead make sure the necessary
methods are defined and that they redirect wherever they need to go.

This is good because it reduces duplication.

This makes me happy!

Define singleton methods on the attributes module instead. This reduces
method pollution on the actual model classes. It also seems to make
something faster, I am unsure why! O_o