Unit `xB` should parsed as `XiB` instead of `XB`

[Bluemangoo]

We found this issue in uutils/procps#667

xB should be XiB, but bytesize ignored cases on parsing unit.

If this would be confirmed as a bug, I can open a pr to fix it. Or we'll apply the fix(uutils/procps#668).

Anyway, please let me know if it will be resolved here.

[robjtede]

bytesize ignored cases

we parse correct form SI and IEC units, "kB" being the correct SI way of expressing decimal kilobytes (ref: https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes)

looks like meminfo is violating this pattern to save 1 character of space in a terminal

it'd be wrong for this library to assume things about application-specific variations