Introduce inline cache for class variable reads¶

@tenderlove and I would like to introduce an inline cache for class variable reads. We've attached a patch that introduces the cache. Class variable reads are popular in Rails applications for example, Active Record's #logger.

GitHub PR: https://github.com/ruby/ruby/pull/4340

Cache Design¶

This patch introduces a hash table that's stored on the same class as the class variable value.

For example:

class A
  @@foo = 1
end

class