isdigit: replace with gnulib's c-ctype

This uses gnulib's c-ctype which ensures that locale will not affect string comparisons.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with other code
• This feature/change has adequate documentation added
• No obvious mistakes in the code