BufStream is universal and can be used with any filesystem so its place is
in different crate. Also moved Partition struct from examples to
fscommon::StreamSlice struct (please note constructor arguments has changed).
Dir methods do not change Dir object itself. They usually clone inner
stream so no change really happens self struct. Underlying partition is
modified but it does not have to affect API. For example see
std::fs::File::set_len. This change greatly simplifies rename API usage.
