String index funcs

[zix99]

Add a few index-functions to operate on strings: index, lastindex, pick, and make 3rd arg on substr optional.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (7fe5873) to head (99e3e65).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   93.16%   93.25%   +0.09%     
==========================================
  Files         147      147              
  Lines        7783     7848      +65     
==========================================
+ Hits         7251     7319      +68     
+ Misses        404      402       -2     
+ Partials      128      127       -1     

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