Fix class name resolution

In new method, where we no long explicitly set loaded class name for
later resolution we depend on file name being same as class name.
However for File output, this is not true, because I didn't want plain
File in code to resolve to Oxidized::File, and din't want to always
write ::File, File ouput has class name OxFile.

This change allows class names to start optionally with string
'Oxidized' if so, they still match.