rename generic io modules

* ext/io/console/console.c, ext/stringio/stringio.c: rename internally
  used modules as generic_readable and generic_writable.  [Fix GH-315]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io/console: rdoc

* ext/io/console/console.c (io_getch): add rdoc.

* ext/io/console/lib/console/size.rb (IO.console_size): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40911 b2dd03c8-39d4-4d8f-98ff-823fe69b080e