Improve GC counting code

[jakobnissen]

When implementing BioJulia/Kmers.jl#48, I want to use BioSequences internals. Keep an open branch here for all optimisations I want to upstream.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.86%. Comparing base (95d9218) to head (36b00f4).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   90.87%   91.86%   +0.99%     
==========================================
  Files          31       29       -2     
  Lines        2400     2804     +404     
==========================================
+ Hits         2181     2576     +395     
- Misses        219      228       +9     

Flag	Coverage Δ
unittests	91.86% <100.00%> (+0.99%)	⬆️

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[kescobo]

Any of this that can be shunted off to BioGenerics.jl to avoid the need to duplicate? Or do you mean that BioSequences is a dependency of Kmers, and you want to make the change here instead of there?

[jakobnissen]

Yeah, I want to upstream some code from Kmers to BioSequences