feat(frontends/lean/parser): add parser_binder(s) and abstract methods

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): add parse_notation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean/run): add new test group, where we just execute Lean (and don't check output)

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