Major changes:
* added Dir::rename method
* optimized FAT access by remembering last allocated cluster (on FAT32 it is also persisted in FSInfo sector)
* added proper generation of unique short names
* made filesystem struct generic so it is no longer fixed to ReadWriteSeek reference usage
* added read_status_flags and stats methods on FileSystem for getting general information about volume
* added FileSystem::unmount method for safe unmounting
* added no_std support