[#3866] Rename the method metaclass to singleton_class to avoid the deprecation warnings from Rails ActiveSupport
The metaid.rb file came straight from why the lucky stiff's "seeing
metaclasses clearly" article. Rails used this too, but they recently
deprecated the name metaclass in favor of singleton_class to match what
ruby-core decided to do. meta, eigen and singlton class were all
suggested and in the end singleton was agreed upon.