packet lib to string: inherits StringifyMixin

Some class inherits stringify.StringifyMixin

and, remove class variable 'tlvs' (not used as class variable
and interferes to string) from lldp.lldp

Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib to string: add __str__ to base

Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib to string: unit tests

Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>