[wp] more robust script session

src/plugins/wp/Changelog

src/plugins/wp/GuiGoal.ml

src/plugins/wp/GuiProof.ml

src/plugins/wp/GuiTactic.ml

src/plugins/wp/ProofScript.ml

src/plugins/wp/ProofSession.ml

src/plugins/wp/ProofSession.mli

src/plugins/wp/ProverScript.ml

src/plugins/wp/ProverSearch.ml

src/plugins/wp/doc/manual/wp_simplifier.tex

src/plugins/wp/register.ml

...f/unroll.0.session/script/unrolled_loop_ensures_zero.json

...in/oracle_qualif/unsigned.0.session/script/lemma_U32.json

...ifiers.0.session/script/split_ensures_Goal_Exist_And.json

...rs.0.session/script/split_ensures_Goal_Exist_And_bis.json

...tifiers.0.session/script/split_ensures_Goal_Exist_Or.json

...ifiers.0.session/script/split_ensures_Hyp_Forall_And.json

...ers.0.session/script/split_ensures_Hyp_Forall_Or_bis.json

...wp/tests/wp_typed/oracle_qualif/user_bitwise.1.res.oracle