Fix parallel fold with StepRange{UInt64, UInt64} etc.

[codecov[bot]]

Codecov Report

Merging #433 into master will decrease coverage by 1.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   91.63%   90.50%   -1.14%     
==========================================
  Files          26       26              
  Lines        1817     1706     -111     
==========================================
- Hits         1665     1544     -121     
- Misses        152      162      +10     

Flag	Coverage Δ
#unittests	90.50% <ø> (-1.14%)	:arrow_down:

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

Impacted Files	Coverage Δ
src/reduce.jl	84.55% <ø> (-1.17%)	:arrow_down:
src/basics.jl	90.00% <0.00%> (-5.32%)	:arrow_down:
src/air.jl	80.00% <0.00%> (-3.34%)	:arrow_down:
src/groupby.jl	82.14% <0.00%> (-2.86%)	:arrow_down:
src/progress.jl	89.79% <0.00%> (-2.52%)	:arrow_down:
src/core.jl	86.09% <0.00%> (-1.54%)	:arrow_down:
src/nondeterministic_threading.jl	84.44% <0.00%> (-1.43%)	:arrow_down:
src/broadcasting.jl	88.88% <0.00%> (-1.12%)	:arrow_down:
src/processes.jl	93.03% <0.00%> (-1.09%)	:arrow_down:
... and 15 more

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[tkf]

2020-10-18T23:07:57.1551784Z Test Summary:     | Pass  Total
2020-10-18T23:07:57.1552568Z test_mapreduce.jl |    8      8
2020-10-18T23:07:57.6508000Z Test Summary:                | Pass  Total
2020-10-18T23:07:57.6508609Z test_might_return_reduced.jl |   12     12
2020-10-18T23:55:29.6845497Z ##[error]The operation was canceled.

--- https://github.com/JuliaFolds/Transducers.jl/runs/1272390831?check_suite_focus=true#step:11:177