Fix range tests on Julia master

[sostock]

Some range calls that would create a StepRange or UnitRange now create a StepRangeLen instead. This PR adapts the tests, adds implementations for Base.range_start_length and Base.range_stop_length and simplifies the range tests.

[codecov-commenter]

Codecov Report

Merging #44 (40a09dd) into master (4923741) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #44   +/-   ##
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  Coverage   99.20%   99.20%           
=======================================
  Files           1        1           
  Lines         250      250           
=======================================
  Hits          248      248           
  Misses          2        2           

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