`round_sf_with_strategy` returns values which exceed the max allowed scale

[extremeandy]

It appears the round_sf_with_strategy does not do appropriate checking on scale and can return invalid values.

Example:

let value = Decimal::new(1, 20)
    .round_sf_with_strategy(12, RoundingStrategy::MidpointAwayFromZero)
    .unwrap();

assert!(value.scale() <= 28);
print!("{value}"); // Also panics with error: 'CapacityError: insufficient capacity'

Perhaps the fix is as simple as capping the precision when doing the rescale? i.e.:

ops::array::rescale_internal(&mut array, &mut value_scale, (scale + digits - mantissa_sf).min(MAX_PRECISION_U32));

I also noted that the return type of round_sf_with_strategy is Option<Decimal>, and the docs say: "If the resulting number is unable to be represented by the Decimal number then None will be returned.". However, all code paths currently return Some.