Add opam metadata

This is a suggestion of opam metadata. It allows to install parsifal on a system with opam using
````
git clone https://github.com/ANSSI-FR/parsifal.git
cd parsifal
opam pin add parsifal .
````
If you intend to publish parsifal on the opam repository, it will be close to complete with that. It will only lack of an 'url' file containing the tarball url and md5sum.