Serialization for NipopowProof

[kettlebell]

Closes #490

[coveralls]

Pull Request Test Coverage Report for Build 2159230004

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 13 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.07%) to 87.629%

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Files with Coverage Reduction	New Missed Lines	%
ergo-p2p/src/peer_info.rs	1	68.42%
ergo-p2p/src/peer_spec.rs	6	82.61%
ergotree-ir/src/serialization/types.rs	6	82.58%

Totals
Change from base Build 2139502449:	-0.07%
Covered Lines:	14606
Relevant Lines:	16668

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💛 - Coveralls

[greenhat]

Looking great! Thank you!