emitter.

* lib/yaml/stream.rb: ditto.

* lib/yaml/rubytypes.rb: added Object#yaml_new.

* lib/yaml/tag.rb: the tag_subclasses? method now
  shows up in the class.  allow taguri to be set using an accessor.
  continue support of Object#to_yaml_type.

* ext/syck/rubyext.c: new emitter code.  yaml_new and yaml_initialize
  get called, should they be present.  consolidated all the diaspora of internal
  node types into the family below YAML::Syck::Node -- Map,
  Seq, Scalar -- all of whom are SyckNode structs pointing to
  Ruby data.  moved Object#yaml_new into the node_import and made it the
  default behavior.  the target_class is always called wih yaml_new, prepended
  a parameter, which is the klass.  loaded nodes through GenericResolver show their style.
  new Resolver#tagurize converts type ids to taguris.

* ext/syck/implicit.re: were 'y' and 'n' seriously omitted??

* ext/syck/emitter.c: renovated emitter, walks the tree in advance.
  consolidated redundant block_styles struct into
  the scalar_style struct.  (this means loaded nodes can now
  be sent back to emitter and preserve at least its very basic
  formatting.)

* ext/syck/gram.c: headless documents of any kind allowed.

* ext/syck/node.c: new syck_replace_str methods and syck_empty_*
  methods for rewriting node contents, while keeping the ID
  and other setup info.  added syck_seq_assign.

* ext/syck/syck.h: reflect block_styles and new node functions.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@9141 b2dd03c8-39d4-4d8f-98ff-823fe69b080e