Improved string safety, and slight performance considerations

[jakewilliami]

• Improved string safety by using SubString, prevind, nextind, firstindex, and lastindex where possible.
• Made performance improvements by changing findfirst(isequal(c), str) to findfirst(c, str) (no allocating, much faster), and by searching for isdigit instead of using regex within addcommas. Using SubString also seemed to be faster.

All tests passing. Please let me know if you need anything else :slightly_smiling_face: