Add Integral.modulo

[jvasileff]

In Ceylon, the result of remainder() takes the sign of the dividend:

-10 % 7 = -3

An alternate interpretation is sometimes useful, where the result is always positive:

-10 mod 7 = 4

For reference, Java's BigInteger offers both remainder and mod.

For completeness and consistence with Whole, it may make sense to add Integral.modulo. Description/possible implementation:

        if (!modulus.positive) {
            throw AssertionError("modulus must be positive");
        }
        return let (result = remainder(modulus))
               if (result.negative)
               then result + modulus
               else result;

[FroMage]

+1

[gavinking]

+1

[FroMage]

Done.