mod(a, -b) does not agree with Julia/AbstractAlgebra

[wbhart]

When the modulus is negative, Nemo always returns a positive remainder. This is inconsistent with what Julia or AbstractAlgebra does. We should switch to the Julia definition.

For example, mod(2, -3) returns -1 in Julia and 2 in Nemo. The Julia definition is better because if a = 2 and m = -3 we have:

a = mq + r where q = -1 and r is between 0 and m.

And just for fun:

mod(ZZ(2), -3)

currently returns 0ffffffffffffffff which wouldn't be my favourite answer.