ErgoTree interpreter, prover, verifier for P2PK tree

[greenhat]

Ref #11

This PR drafts all steps of the prover. All unimplemented parts are listed in #11 TODO section.

[coveralls]

Pull Request Test Coverage Report for Build 484

• 61 of 246 (24.8%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-7.4%) to 67.621%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sigma-tree/src/serialization/sigmaboolean.rs	0	1	0.0%
sigma-tree/src/ergo_tree.rs	5	7	71.43%
sigma-tree/src/eval/cost_accum.rs	0	5	0.0%
sigma-tree/src/sigma_protocol/dlog_group.rs	42	47	89.36%
sigma-tree/src/sigma_protocol/dlog_protocol.rs	0	7	0.0%
sigma-tree/src/sigma_protocol/verifier.rs	0	13	0.0%
sigma-tree/src/sigma_protocol.rs	2	20	10.0%
sigma-tree/src/eval.rs	0	28	0.0%
sigma-tree/src/sigma_protocol/prover.rs	0	106	0.0%

Files with Coverage Reduction	New Missed Lines	%
sigma-tree/src/sigma_protocol.rs	1	43.59%

Totals
Change from base Build 469:	-7.4%
Covered Lines:	1063
Relevant Lines:	1572

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💛 - Coveralls