fixes #13

fixes #18

This allows int-vectors to be multiplied by float-scalars, producing
accurate int values (e.g. 4.2 * 10 == 42).

According to benchmarks, there is practically no difference in
performance. Only explicit unrolling is helpful, but that has been
implemented for the general algorithm in Reduction::reduce as well.

Now that error handling is done via exceptions, the return value is free
for more sensible things.