Basic versioning

It would be desirable to implement a basic versioning along with -version option to simply fetch the current OVM version.

Dune package may be used to specify a version, in addition to nix flake.

For instance:

$ havm --version

HAVM 0.28
Written by Robert Anisko.

Copyright (C) 2002-2003  Robert Anisko
Copyright (C) 2003-2007, 2009, 2011-2014  EPITA Research and Development Laboratory (LRDE).
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.