Split multibyte class names from Marshal fixtures into their own file

[herwinw]

This removes the dependency on module_eval. It does use the magic comment to force the file encoding, but that one is present in the main fixture as well.

[andrykonchin]

Yeah, makes sense. Thank you!

[andrykonchin]

The only concern I have is that a file in the fixtures directory requires another sibling file. I would place the new file into a subdirectory. Actually the spec/ruby/core/marshal/fixtures/marshal_data.rb file's content is what usually stored in classes.rb files (in <class>Spec module).