rb_ to avoid name clash in writing extensions.

* ext/digest: Introduce Digest::Class and Digest::Instance for
  ease of implementing subclasses and add-ons, inspried by
  gotoyuzo.

* ext/digest: The Digest::Instance module now requires and assumes
  that any instance be resettable and clonable, and add some
  convenient instance methods such as "new()", for creating a new
  copy, parameter taking "digest()" and "hexdigest()", for instant
  calculation.  These methods make digest instances work just like
  digest classes.

* ext/digest/sha2/lib/digest/sha2.rb:
  Add the Digest::SHA2 class to wrap up SHA2 variants: SHA256,
  SHA384 and SHA512, hoping this module would make a decent
  example of a digest subclass written in Ruby.

* ext/digest/lib/digest.rb: Adjust autoload entries for SHA2
  classes.

* ext/digest/lib/digest/hmac.rb: Follow the framework updates.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@11197 b2dd03c8-39d4-4d8f-98ff-823fe69b080e