cgo: Properly manage memory passing from cgo to go on Batch Verifiers

[algonautshant]

Fixing https://github.com/algorand/go-algorand-internal/issues/3011

1. Prepare a contiguous memory block for messages, without additional copying
2. Use the provided wrapper C function to avoid the use of unsafe pointers

From #5695:

This updates the batchVerifier and its enqueue function to build up a 1-dimensional byte slice, rather than a slice of slices or a slice of arrays, for passing to the C batch verification code. This removes the need to build them on demand in batchVerificationImpl. A new C function acts as a shim between Go and C.

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (b19594e) 55.25% compared to head (a50dc52) 55.24%.
Report is 112 commits behind head on master.

Files	Patch %	Lines
crypto/curve25519.go	33.33%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5700      +/-   ##
==========================================
- Coverage   55.25%   55.24%   -0.01%     
==========================================
  Files         473      473              
  Lines       66166    66182      +16     
==========================================
+ Hits        36558    36565       +7     
- Misses      27146    27148       +2     
- Partials     2462     2469       +7     

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[jannotti]

Looks right to me.