to_index for RealInfinity

[jishnub]

Since RealInfinity is being used as an index in https://github.com/JuliaArrays/InfiniteArrays.jl/blob/e323a66574662ae4b171cd8fc6822ad79e27ea8c/src/infarrays.jl#L119-L120, it makes sense to define to_index for it instead of extending getindex.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8580d78) 99.19% compared to head (bc014ed) 92.99%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   99.19%   92.99%   -6.20%     
==========================================
  Files           2        2              
  Lines         373      357      -16     
==========================================
- Hits          370      332      -38     
- Misses          3       25      +22     

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