unix: Add "_os" module with stat().

stat() is bad function to use using FFI, because its ABI is largely private.
To start with, Glibc .so doesn't even have "stat" symbol. Then, layout of
struct stat is too implementation-dependent. So, introduce _os to deal
with stat() and other similar cases.
objstringio: Implement io.BytesIO.

Done in generalized manner, allowing any stream class to be specified as
working with bytes.