Improve type stability slightly

[jakobnissen]

Remove a few uncaught type instabilities.

[codecov[bot]]

Codecov Report

Merging #88 (ff2f256) into master (f59007a) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   89.98%   90.03%   +0.04%     
==========================================
  Files          11       11              
  Lines         629      632       +3     
==========================================
+ Hits          566      569       +3     
  Misses         63       63              

Flag	Coverage Δ
unittests	90.03% <100.00%> (+0.04%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/fasta/index.jl	85.41% <100.00%> (+0.10%)	:arrow_up:
src/fasta/reader.jl	87.91% <100.00%> (+0.27%)	:arrow_up:

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