transcode.c: add rb_econv_append

* transcode.c (rb_econv_append): new function to append a string data
  with converting its encoding.  split from rb_econv_substr_append.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42853 b2dd03c8-39d4-4d8f-98ff-823fe69b080e