Fix typo (thx to @rixed)
Implement a refinment loop in the solver heuristic.

The firt side-effect of this change is to improve the minimality of installed packages. I guess it could help in other situations as well (and at least, it cannot make things worse).
Only refine the request if it is valid to do so (eg. if there are no constraint in the initial request).

Note: we could refine if the initial request already have a `Geq, but not sure it's worth.
Fix build on 3.12.1 by upgrading ocp-build to OCamlPro/typerex@1fa4aeb21f7848891646e2107cd2c6991f4e85ed
Add state saving stats
Fix #567

'opam upgrade <package>' was only working for packages in the reinstall list. Now it should work for all installed packages.
Fix stupid typo (would be nice if cmdliner were able to spot these ambiguities)

Fix #575
Update HTML doc
Substitute version values in generated man-page for opam-* scripts

(this is related to #561)
Update configure files to substitute the man-pages
Update the generated ./configure
Fix #561

(was missing the -s option, to generate stand-alone man pages)
Update the generated man pages
Update the tutorials
Add a link to the online documentation in README.md
Improve the documentation for OpamHeuristic
Update HTML doc for OpamHeuristic
Improve the documention of OpamHeuristic (cont)
Fix typos in previous commits, rework some sentences.
Update API doc