Cleanup Part #1 of IO::Handle

- no longer does IO::FileTestable, has its own "handlers" now
- added support for IO::Special paths
- simplified a lot of unneccesary nqp::bindattr calls
IO::Path::Special -> IO::Special

No need to inherit from IO::Path, so this is a better name
Fix IO::Path.open
Fix open() and initialization of $*IN|OUT|ERR