feat(util/rb_tree): add max (element) method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/info_manager): several bugs: invalid flag was not being reset for empty lines, merge with overwrite=false was adding 'poluting' state, --NAY generation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/info_manager): several bugs: invalid flag was not being reset for empty lines, merge with overwrite=false was adding 'poluting' state, --NAY generation