OCaml API: small fix and test updates for K-256

[victor-dumitrescu]

This MR contains:

• a fix for Libsecp256k1.normalize_signature, which, in addition to returning the normalized signature as an option, was also writing it in-place in the input buffer
• more precise testing for K-256 / libsecp256k1 signatures

[coveralls]

Pull Request Test Coverage Report for Build 4928042237

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 53.441%

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Totals
Change from base Build 4925587254:	0.02%
Covered Lines:	30398
Relevant Lines:	56881

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💛 - Coveralls