Allow negative exponents for GroupElement.exp

[SethDusek]

This fixes validation for transaction 307696f33cc8b03ac731616f025cb33555f854ddb3ab58cfd8af884c59225cd1 and chaincash schnorr signature verification

[coveralls]

Pull Request Test Coverage Report for Build 9823666223

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.06%) to 79.0%

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Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/eval/exponentiate.rs	1	66.67%
ergotree-ir/src/serialization/types.rs	8	81.88%

Totals
Change from base Build 9822292740:	0.06%
Covered Lines:	10804
Relevant Lines:	13676

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💛 - Coveralls