Rework ParseBytes to return uint64 instead of interface

[martialblog]

Fixes #147

[RincewindsHat]

hm, wouldn't it make sense to remove the types BytesSI and BytesIEC completely and replace them with a simple formatter function which receives an argument whether IEC or SI is wished for?
Sure, there is no String interface there anymore, but it feels like a lot of overhead atm.

[martialblog]

Yeah I think you're right. We can simplify that.

[martialblog]

@RincewindsHat I squashed some things together while keeping the same functionality

[RincewindsHat]

Looks good now, minmal and pretty clear.

Should we keep the Bytes = uint64 alias to make it more obvious that you get that from the one function and put it into the other?
Or is it plainly obvious that the uint64 is the number of bytes?

[martialblog]

I think uint64 and clear documentation is fine.

[martialblog]

@RincewindsHat squashed the commits, added some examples to the docs.