custom Menhir error reporting does not work in TryWhy3

report.ml uses functions from Str (search_forward), which are not implemented in js_of_ocaml. Thus, instead of "syntax error", TryWhy3 shows "Failure: re_search_forward not implemented". Removing the call to Report.report from lexer.mll fixes the problem, but we need a proper solution to this.

(I am amazed that TryWhy3 manages to avoid other uses of Str regexps — we seem to have quite a number of them in Why3.)