Add: [sys] sys.null() to obtain writable stream for system null device.
Change: [binary] exclude bool arrays.
Add: [os.fs] fs::File.copy() accepting fs::Dir.
Minor fix: [os.fs] docs.
Minor change: [sys] sys.pclose() according to the recent changes with io::Stream.
Change: move module "sys" to module "os";
Change: a few time methods to return the self object.