- Introduce versioning in Digest::Base API, and prefix C
    constants with RUBY_ and C type names with rb_ to avoid name
    clash in writing extensions.
  - Introduce Digest::Class and Digest::Instance for ease of
    implementing subclasses and add-ons.
  - Digest::Instance module requires and assumes that any instance
    be resettable and clonable.  An instance method #new() is
    added so digest instances work just like digest classes.
  - The constructor does no longer take an initial string to feed;
    digest() and hexdigest() now do, instead.  This allows digest
    classes to take their own hashing parameters.
  - Make some changes to digest() and hexdigest() class methods,
    which now take extra arguments, which are passed through to
    the constructor in an internal call.
  - Add #digest_length/size/length() and #block_length(),
  - 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.
  - Rip BubbleBabble support out of the base class and have a
    separate module named digest/bubblebabble.
  - Remove RD documents in favor of newly written and embedded
    RDoc documentation.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@11223 b2dd03c8-39d4-4d8f-98ff-823fe69b080e