Add quality and sequence iterator

[jakobnissen]

As described in #40 , this adds a small PR to obtain the sequence and quality from FASTA/FASTQ files without allocating a new vector.

[codecov]

Codecov Report

Merging #41 (72aadd2) into master (b1a8ecb) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   87.51%   87.73%   +0.21%     
==========================================
  Files          14       14              
  Lines         633      644      +11     
==========================================
+ Hits          554      565      +11     
  Misses         79       79              

Flag	Coverage Δ
unittests	87.73% <100.00%> (+0.21%)	⬆️

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Impacted Files	Coverage Δ
src/fasta/fasta.jl	100.00% <ø> (ø)
src/fasta/record.jl	79.71% <100.00%> (+0.76%)	⬆️
src/fastq/record.jl	83.45% <100.00%> (+0.78%)	⬆️

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