Support parsing EB/EiB

[tisonkun]

One EB is 1152921504606846976 bytes, less than u64::MAX that can be represented.

More important, our Display supports EiB and thus:

  let bytes = bytesize::ByteSize::from_str("16384 PiB").unwrap();
  let bytes = bytesize::ByteSize::from_str(&format!("{bytes}")).unwrap();

Will panic in the second from_str because EiB is not supported.

cc @hyunsik @robjtede

I'm glad to prepare a patch if this is valid.