remove Iterators.jl

[ChrisRackauckas]

It is causing a bunch of problems

JuliaLang/julia#21969

and it only needed one function.

[codecov-io]

Codecov Report

Merging #49 into master will decrease coverage by 0.54%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   96.96%   96.42%   -0.55%     
==========================================
  Files           6        7       +1     
  Lines         594      616      +22     
==========================================
+ Hits          576      594      +18     
- Misses         18       22       +4

Impacted Files	Coverage Δ
src/iterators.jl	81.81% <81.81%> (ø)

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[JeffBezanson]

Base.Iterators.flatten also provides this functionality.

[musm]

The only issue to determine is what to do with the [] case

Using Base.Iterators.flatten
[combinations([])...]
gives
ERROR: ArgumentError: argument to Flatten must contain at least one iterator
whereas currently with chain
[combinations([])...] == []

[ChrisRackauckas]

Julia v0.6 has been released. Can we merge some fix, even if it's not this one?

[musm]

see #50 , we should probably close this in favor of that PR