Replace num256 with bnum

[SethDusek]

num256 has no no_std support and uses BigInt internally, so it'll be less efficient for fixed-size ops compared to bnum

Part of #778

[coveralls]

Pull Request Test Coverage Report for Build 11835992403

Details

• 65 of 70 (92.86%) changed or added relevant lines in 7 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.005%) to 78.658%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-ir/src/bigint256.rs	44	49	89.8%

Files with Coverage Reduction	New Missed Lines	%
ergotree-ir/src/sigma_protocol/sigma_boolean.rs	1	69.03%
ergo-p2p/src/peer_database/in_memory.rs	1	86.36%
ergotree-ir/src/serialization/types.rs	6	82.58%

Totals
Change from base Build 11817168621:	0.005%
Covered Lines:	11009
Relevant Lines:	13996

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💛 - Coveralls