add 0-1 vector

[aplavin]

@tkf: seems like such a type would be in scope of MicroCollections. If this addition is fine, I'll add tests.

[codecov]

Codecov Report

Merging #16 (bd17c64) into master (ad32b16) will decrease coverage by 2.29%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   85.37%   83.07%   -2.30%     
==========================================
  Files           8        8              
  Lines         253      260       +7     
==========================================
  Hits          216      216              
- Misses         37       44       +7     

Flag	Coverage Δ
unittests	83.07% <0.00%> (-2.30%)	⬇️

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

Impacted Files	Coverage Δ
src/vectors.jl	82.35% <0.00%> (-13.11%)	⬇️

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