Add SUnitRange(::Int, ::Int) constructor and doc string

[Tokazama]

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[codecov]

Codecov Report

Merging #223 (301d3e6) into master (1966706) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   87.75%   87.71%   -0.05%     
==========================================
  Files          11       11              
  Lines        1780     1782       +2     
==========================================
+ Hits         1562     1563       +1     
- Misses        218      219       +1     

Impacted Files	Coverage Δ
src/ranges.jl	91.30% <100.00%> (-0.33%)	⬇️

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