[6.0] Fix for downcasting BigInt to other numeric types

[SethDusek]

For #770. Also fixes upcasting BigInt256 to BigInt256 (previously sigma-rust would accept this on all versions, even though this is 6.0-specific)

[coveralls]

Pull Request Test Coverage Report for Build 11177535395

Details

• 41 of 45 (91.11%) changed or added relevant lines in 3 files are covered.
• 20 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 79.007%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-interpreter/src/eval/downcast.rs	32	33	96.97%
ergotree-interpreter/src/eval/upcast.rs	4	5	80.0%
ergotree-ir/src/bigint256.rs	5	7	71.43%
<!--	Total:	41	45	91.11%	-->

Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/eval/upcast.rs	2	77.78%
ergotree-ir/src/serialization/types.rs	8	82.58%
ergotree-interpreter/src/eval/downcast.rs	10	85.42%
<!--	Total:	20		-->

Totals
Change from base Build 11146473813:	-0.02%
Covered Lines:	10884
Relevant Lines:	13776

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💛 - Coveralls