fix(src/emacs/lean-syntax.el): remove obsolete command set_opaque

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(src/emacs/lean-syntax.el): add commonly used operators

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library): minor cleanup

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>