abacus: bignum: trim zeros after a subtraction

This way we keep a canonicalized version of the result.

src/bignum/bignum.cc

@@ -82,6 +82,8 @@ digits_type do_substraction(digits_type const& lhs, digits_type const& rhs) {
     std::transform(complement.begin(), complement.end(), complement.begin(),
                    take_complement);
 
+    do_trim_leading_zeros(complement);
+
     return complement;
 }