crypto: convert BatchVerifier to interface

[cce]

Summary

This changes the BatchVerifier to be an interface type, with a cgoBatchVerifier implementation, with a goal towards enabling an alternate pure Go implementation later in a future PR.

Test Plan

Existing tests should pass.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 56.14%. Comparing base (5f81b7b) to head (b9b8113).
Report is 3 commits behind head on master.

Files	Patch %	Lines
crypto/onetimesig.go	63.63%	3 Missing and 1 partial ⚠️
crypto/batchverifier.go	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5988      +/-   ##
==========================================
+ Coverage   52.42%   56.14%   +3.72%     
==========================================
  Files         482      482              
  Lines       67969    67992      +23     
==========================================
+ Hits        35630    38173    +2543     
+ Misses      29656    27224    -2432     
+ Partials     2683     2595      -88     

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