use instance_eval, reduce method calls.

execution time in seconds for `rake test_sqlite3`:

ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-darwin10.4.0]
  - before: 52.771426, 52.159505, 52.937056
  - after: 51.452584, 51.897017, 51.584431

rubinius 1.0.1 (1.8.7 release 2010-06-03 JI) [x86_64-apple-darwin10.4.0]
  - before: 284.334076
  - after: 285.753028

ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]
  - before: 75.811548, 75.533153, 75.353122
  - after: 75.244243, 75.19226, 75.256925

activerecord/lib/active_record/base.rb

@@ -884,21 +884,13 @@ def relation #:nodoc:
         # single-table inheritance model that makes it possible to create
         # objects of different types from the same table.
         def instantiate(record)
-          object = find_sti_class(record[inheritance_column]).allocate
-
-          object.instance_variable_set(:@attributes, record)
-          object.instance_variable_set(:@attributes_cache, {})
-          object.instance_variable_set(:@new_record, false)
-          object.instance_variable_set(:@readonly, false)
-          object.instance_variable_set(:@destroyed, false)
-          object.instance_variable_set(:@marked_for_destruction, false)
-          object.instance_variable_set(:@previously_changed, {})
-          object.instance_variable_set(:@changed_attributes, {})
-
-          object.send(:_run_find_callbacks)
-          object.send(:_run_initialize_callbacks)
-
-          object
+          find_sti_class(record[inheritance_column]).allocate.instance_eval do
+            @attributes, @attributes_cache, @previously_changed, @changed_attributes = record, {}, {}, {}
+            @new_record = @readonly = @destroyed = @marked_for_destruction = false
+            _run_find_callbacks
+            _run_initialize_callbacks
+            self
+          end
         end
 
         def find_sti_class(type_name)