Fix findfirst for InfStepRange for negative step

[jishnub]

Fixes #180

After this,

julia> findfirst(==(9), 10:-1:-∞)
2

julia> findfirst(==(11), 10:-1:-∞)

julia> findfirst(==(21), 10:-1:-∞)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.14%. Comparing base (8865985) to head (227ef65).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   85.49%   86.14%   +0.65%     
==========================================
  Files           7        6       -1     
  Lines         772      758      -14     
==========================================
- Hits          660      653       -7     
+ Misses        112      105       -7     

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