Multiplication underflow should be rounded instead of panicing about rust-decimal

[rkusa]

Here is an example. The numbers from #89 could be re-used: 0.6386554621848739495798319328 * 11.815126050420168067226890757 will throw Multiplication underflowed.

I've created a smaller example to show how we could end up with such numbers. While I purposely build this example to panic, I sometimes encounter this panic after long sequences of "real" operations.

let rate = Decimal::new(19, 2);
let part = rate / (rate + Decimal::new(1, 0));

let a = Decimal::new(74, 0) * part; // = 11.815126050420168067226890757
let b = Decimal::new(4, 0) * part; // = 0.6386554621848739495798319328
let result = b * a; // thread 'main' panicked at 'Multiplication underflowed: 35 > 28'

Thanks!

[paupino]

Easiest way to reproduce this is with two highly significant numbers multiplying together.
e.g.
2.1234567890123456789012345678 * 2.1234567890123456789012345678 currently panics whereby I'd expect a rounded result of 4.5090687348026215523554336227.

[paupino]

Cool; Thank you! I'll add that test case too. Result here should round to approx 7.5457947885036367488171739292