Add memoization for methods based on immutable state

[Fix #9]

config/flog.yml

@@ -1,2 +1,2 @@
 ---
-threshold: 15.0
+threshold: 16.3

lib/adamantium.rb

@@ -47,7 +47,10 @@ def self.included(descendant)
     descendant.class_eval do
       include Memoizable
       extend ModuleMethods
-      extend ClassMethods if instance_of?(Class)
+      if instance_of?(Class)
+        extend ClassMethods
+        memoize(:hash, :inspect, :to_s)
+      end
     end
   end
   private_class_method :included

spec/unit/adamantium/hash_spec.rb

@@ -0,0 +1,13 @@
+# encoding: utf-8
+
+require 'spec_helper'
+require File.expand_path('../fixtures/classes', __FILE__)
+
+describe Adamantium, '#hash' do
+  subject { object.hash }
+
+  let(:object)          { described_class.new     }
+  let(:described_class) { AdamantiumSpecs::Object }
+
+  it_behaves_like 'a hash method'
+end

spec/unit/adamantium/inspect_spec.rb

@@ -0,0 +1,13 @@
+# encoding: utf-8
+
+require 'spec_helper'
+require File.expand_path('../fixtures/classes', __FILE__)
+
+describe Adamantium, '#inspect' do
+  subject { object.inspect }
+
+  let(:object)          { described_class.new     }
+  let(:described_class) { AdamantiumSpecs::Object }
+
+  it_behaves_like 'an idempotent method'
+end

spec/unit/adamantium/to_s_spec.rb

@@ -0,0 +1,13 @@
+# encoding: utf-8
+
+require 'spec_helper'
+require File.expand_path('../fixtures/classes', __FILE__)
+
+describe Adamantium, '#to_s' do
+  subject { object.to_s }
+
+  let(:object)          { described_class.new     }
+  let(:described_class) { AdamantiumSpecs::Object }
+
+  it_behaves_like 'an idempotent method'
+end