Deprecate isDigit and digitToInt in favor of functions which specify base

[milesfrain]

Description of the change
Fixes #25

Now we have symmetric:

isHexDigit        
isDecDigit        
isOctDigit        
hexDigitToInt        
decDigitToInt        
octDigitToInt

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Added a test for the contribution (if applicable)

Not Applicable:

• Updated or added relevant documentation in the README and/or documentation directory
  • I think the API docs in the function comments are enough in this case.

[thomashoneyman]

Thank you!