[m-dev.] is_digit/1 and digit_to_int/2

[Julien Fischer]

Hi,

I think it is potentially confusing that char.is_digit/1 and
char.digit_to_int/2 are true for different inputs.  The former only
succeeds for 0-9, while the latter succeeds for 0-9a-zA-Z.

At the very least I think we should have is_decimal_digit/1 and
decimal_digit_to_int/2 that are both only true for 0-9.

Cheers,
Julien.