Lazily add Aggregations to composed_of models

`composed_of` is a fairly rare method to call on models.  This commit
adds the `Aggregations` module to models that call `composed_of` so that
models that *don't* call `composed_of` don't need to instantiate the
`aggregation_cache` hash.  This saves one hash allocation per model
instance that doesn't use `composed_of`

activerecord/lib/active_record/aggregations.rb

@@ -225,6 +225,10 @@ module ActiveRecord
         def composed_of(part_id, options = {})
           options.assert_valid_keys(:class_name, :mapping, :allow_nil, :constructor, :converter)
 
+          unless self < Aggregations
+            include Aggregations
+          end
+
           name        = part_id.id2name
           class_name  = options[:class_name]  || name.camelize
           mapping     = options[:mapping]     || [ name, name ]

activerecord/lib/active_record/base.rb

@@ -288,6 +288,7 @@ module ActiveRecord #:nodoc:
     extend Enum
     extend Delegation::DelegateCache
     extend CollectionCacheKey
+    extend Aggregations::ClassMethods
 
     include Core
     include DatabaseConfigurations
@@ -314,7 +315,6 @@ module ActiveRecord #:nodoc:
     include ActiveModel::SecurePassword
     include AutosaveAssociation
     include NestedAttributes
-    include Aggregations
     include Transactions
     include TouchLater
     include NoTouching