Add support for separators

[ecton]

DigitCollection should support automatically inserting separators every X digits. This can't be done automatically for non-digit-based systems, because we can't trust that multiple characters are different digits -- e.g., how would separators work in roman numerals?

[WorkProg]

This might be complicated even more by some locales use non-constant separator spacing, I know of at least Indian https://en.wikipedia.org/wiki/Decimal_separator#Digit_grouping