Utilize `#` specifier to implement `Display` trait for SI unit

[EFanZh]

So that it is possible to use {} for decimal units (KB, MB, GB), and {:#} for binary units (KiB, MiB, GiB).

[wookietreiber]

This seems rather ad-hoc to me. I would prefer explicitly naming the unit rather than having this be done ad-hoc with formatting specifiers.

[loyd]

KB by default looks strange to me. Never have I met a programmer thinking in decimal units instead of binary units.