Optimizing dinucleotides count in a DNA sequence type `LongDNA`

[rocco_sprmnt21]

squeezing a little more…

julia> @btime begin
           #a = @view rseq[begin:end-1]
           b = @view rseq[begin+1:end]
           countmap(zip(rseq, b))
       end
  10.514 ms (11 allocations: 1.41 KiB)