k256: make ecdsa::Signature::normalize_s non-mutating

[tarcieri]

Corresponding changes for RustCrypto/signatures#355

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.60%. Comparing base (05ea002) to head (cfb4c9d).
Report is 671 commits behind head on master.

Files with missing lines	Patch %	Lines
k256/src/ecdsa/recoverable.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   62.61%   62.60%   -0.02%     
==========================================
  Files          28       28              
  Lines        4026     4022       -4     
==========================================
- Hits         2521     2518       -3     
+ Misses       1505     1504       -1     

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