WIP: Add Oligomer

[jakobnissen]

This type adds Oligomer, a kmer backed by a single unsigned integer. The length is encoded in parts of the integer's bits.
This is useful for code using a small range of kmer lengths, which would other- wise cause type instability.
One user could be to represent primers, or immunogenic peptides.

I tried to back oligos with an NTuple like kmers are, but it's very hard to made the code efficient and type stable, so I had to resort to backing it by a single unsigned integer.

TODO

Make Oligomer on level with Kmer.

• Restructure files, such that Oligomer is not in its own little file
• Go through all non-iterator code and check if Oligomer also needs this
• Implement construction utils for Oligos
• Make kmer iterators generic over Kmer / Oligomer

[codecov]

Codecov Report

❌ Patch coverage is 93.56322% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.83%. Comparing base (c6754be) to head (3e9f591).

Files with missing lines	Patch %	Lines
src/dynamic.jl	93.58%	25 Missing ⚠️
ext/BitIntegersExt.jl	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   98.49%   96.83%   -1.67%     
==========================================
  Files          16       18       +2     
  Lines         865     1294     +429     
==========================================
+ Hits          852     1253     +401     
- Misses         13       41      +28     

Flag	Coverage Δ
unittests	96.83% <93.56%> (-1.67%)	⬇️

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