dummy forward-compatibility package for the standard Bytes module

Starting from 4.02, the OCaml standard library splits String into two
modules, String for now-immutable strings and Bytes for mutable byte
sequences. A 'bytes' package is made available to older OCaml version
that wish to use a Bytes module. This dummy 'bytes' package, intended
to be installed on 4.02 and above, makes user-code depending on
'bytes' forward-compatible.
