hexadecimal: Supress HLint's suggestion to use isDigit.
hexadecimal: Supress HLint's suggestion to use isDigit.

The following hint makes the function less readable:

```
./src/Example.hs:5:5: Warning: Use isDigit
Found:
  c >= '0' && c <= '9'
Why not:
  isDigit c

1 hint
```

exercism/xhaskell