relax signbit in range construction

[jishnub]

This allows non-number types to be used in infinite ranges. E.g. infinite range of dates:

julia> InfiniteArrays.InfStepRange(Date(2000, 1, 1), Day(1))
Date("2000-01-01"):Day(1):+∞

[codecov]

Codecov Report

Merging #144 (fa482ee) into master (9360eb6) will decrease coverage by 0.10%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   85.92%   85.83%   -0.10%     
==========================================
  Files           6        6              
  Lines         732      734       +2     
==========================================
+ Hits          629      630       +1     
- Misses        103      104       +1     

Files	Coverage Δ
src/infrange.jl	87.34% <66.66%> (-0.24%)	⬇️

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