fix(emacs/lean-changes): visit file before process any changes

This solves many instances of the modified buffer problem.
feat(emacs/lean-server): delete cache file (.clean) after visit/load

fix #75
feat(emacs/lean-mode): add 'clean-cache' to the menu

Related issue: #75
feat(emacs/lean-server): send SYNC command when visiting a modified buffer

Close #201
chore(tests/lean/run/group3): cleanup test
feat(frontends/lean/notation_cmd): notation defined in context overrides existing ones
refactor(frontends/lean/parser_config): merge notation_ext and mpz_notation_ext
feat(frontends/lean/notation_cmd): allow local numeric notation