[Impl] message validation

[yudetamago]

No description provided.

[yudetamago]

See:

• [Spec] How to check whether some data is valid message or valid state efficiently? cbc-beacon-chain#1 (comment)
• https://github.com/LayerXcom/cbc-casper-simulator/blob/master/cbc_casper_simulator/message_validator.py

[nrryuya]

Same thought with #4 (comment)

[yudetamago]

Makes sense, complete validation is not necessary, but some of simplified validations (like justification_is_justified as described above) are useful for debugging.
Specifically, We might use it as just an assertion for experiments like:

cbc-casper-simulator/cbc_casper_simulator/simulator/broadcast_and_receive_simulator.py

Lines 35 to 36 in de186bd

	res = sender.add_message(message)
	assert res.is_ok(), res.value

[yudetamago]

About justification_is_justified, it is required to check whether the message is to be "pending" or not.