SI unit should be decimal units

[EFanZh]

Currently, the effect of the si_unit argument of ByteSize::to_string_as seems to be reversed. According to Wikipedia, SI units are decimal units, and IEC units are binary units.

[wookietreiber]

I highly recommend using an enum with SI and IEC variants instead of a bool, as this is so much clearer in code.

[MrCroxx]

Planned with 2.0 🥰