add DHT variant and JSON encoding for SecretKey;

[greenhat]

Close #664

[coveralls]

Pull Request Test Coverage Report for Build 3793592757

• 44 of 53 (83.02%) changed or added relevant lines in 2 files are covered.
• 28 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.2%) to 82.834%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergo-lib/src/wallet/secret_key.rs	19	28	67.86%

Files with Coverage Reduction	New Missed Lines	%
ergo-lib/src/wallet/secret_key.rs	2	71.05%
ergo-merkle-tree/src/merkletree.rs	2	97.39%
ergo-rest/src/api/peer_discovery_internals/non_chrome.rs	2	72.52%
ergo-merkle-tree/src/batchmerkleproof.rs	3	90.48%
ergotree-ir/src/chain/token.rs	5	72.58%
ergotree-ir/src/serialization/types.rs	14	80.07%

Totals
Change from base Build 3766480790:	-0.2%
Covered Lines:	9892
Relevant Lines:	11942

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💛 - Coveralls