Problem:
- Post-increment of loop index.
- `const` variables are not marked `const`.
- Incorrect type for loop index.
Solution:
- Pre-increment loop index.
- Mark all possible variables `const`.
- Corret type for loop index.
Problem:
- Hex digit decoding is not `constexpr`, but can be.
Solution:
- Move the body of the function to the header and decorate with
`constexpr`.
- Provide tests for run-time and compile-time evaluation.
