Extend the os module

Currently, the python os module is missing many methods. Try adding a method to this module by editing the file vm/src/stdlib/os.rs.

This is an issue for new contributors, so please do not implement everything at once, so other people have a chance to go for it (or, actually, please implement all! >:)! ).

Note: Before working on platform-specific functions, check if they are members of posix or nt. Their members are reexported to os.