[wp] refactor prover stats

src/plugins/wp/Cache.ml

src/plugins/wp/ProofEngine.ml

src/plugins/wp/ProofScript.ml

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/Stats.ml

src/plugins/wp/Stats.mli

src/plugins/wp/VCS.ml

src/plugins/wp/VCS.mli

src/plugins/wp/gui/GuiList.ml

src/plugins/wp/gui/GuiProver.ml

src/plugins/wp/register.ml

...gins/wp/tests/wp_gallery/oracle_qualif/bsearch.res.oracle

...ugins/wp/tests/wp_gallery/oracle_qualif/euclid.res.oracle

...lugins/wp/tests/wp_plugin/oracle_qualif/doomed.res.oracle

...wp/tests/wp_plugin/oracle_qualif/doomed_axioms.res.oracle

...wp/tests/wp_plugin/oracle_qualif/doomed_call.1.res.oracle

...wp/tests/wp_plugin/oracle_qualif/doomed_call.2.res.oracle

...wp/tests/wp_plugin/oracle_qualif/doomed_dead.0.res.oracle

...wp/tests/wp_plugin/oracle_qualif/doomed_dead.1.res.oracle

...tests/wp_plugin/oracle_qualif/doomed_localinit.res.oracle