Ruby

• include/ruby/encoding.h (rb_econv_result_t): moved from
  transcode_data.h.
  (rb_econv_elem_t): ditto.
  (rb_econv_t): ditto. source_encoding and destination_encoding field
  is added.
  (rb_econv_open): declared.
  (rb_econv_convert): ditto.
  (rb_econv_close): ditto.

• transcode.c (rb_econv_open_by_transcoder_entries): initialize
  source_encoding and destination_encoding field as NULL.
  (rb_econv_open): make it external linkage.
  (rb_econv_close): ditto.
  (rb_econv_convert): ditto. renamed from rb_econv_conv.
  (make_encoding): new function.
  (econv_init): use make_encoding and store rb_encoding* in
  rb_econv_t.
  (econv_source_encoding): new method
  Encoding::Converter#source_encoding.
  (econv_destination_encoding): new method
  Encoding::Converter#destination_encoding.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18625 b2dd03c8-39d4-4d8f-98ff-823fe69b080e