Memoize methods that depend on instance state automatically

[dkubb]

An immutable object's state never changes, so the #hash, #inspect and #to_s methods are idempotent. These methods should be memoized by default.

[mbj]

I like this change.

We have to communicate methods need to be present / changed before Adamantium is included. Especially in conjunction with Equalizer.

Correct:

class Foo
  include Adamantium, Equalizer.new(:foo)
end

Incorrect:

class Foo
  include Equalizer.new(:foo), Adamantium
end

Module#include does include modules in reverse order.

[dkubb]

@mbj sure, the first thing we could do show examples using include on separate lines, then it's more clear the ordering of things. We can have an "advanced" section showing this one-liner, which includes Equalizer first then Adamantium.

[mbj]

I just remember a chat conversation in #jruby, we improved performance on rack hashes with using separate String subclasses for keys that freezed contents and cachesObject#hash I lost the gist the guy created but we had around 20% speedup.

[dkubb]

@mbj wdyt about #25 ?