fix(library/definitional/no_confusion): assertion violation
feat(emacs/lean-syntax): highlight [decls] modifier
feat(frontends/lean): default 'eval' command ignores opaque/irreducible annotations

To retrieve the previous behavior, we should use [strict] modifier
test(tests/lean/run): fibonacci using well_founded recursion
refactor(library/data/nat/wf): mark theorem as transparent

It doesn't really help since
        le_imp_lt_or_eq, succ_le_cancel, lt_imp_le_succ and or.elim
are still opaque
feat(library/logic/wf): some basic definitions for constructing well_founded relations