计算字节单位

```rust

[cfg(test)]

mod tests { use std::ops::Sub; use crate::size::{add, ByteSize, ByteSizeUnit}; use std::str::FromStr; use super::*;

#[test]
fn parse() {
    assert_eq!(ByteSize::from_str("12.3 PB").unwrap().to_size(), ByteSize::pb(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 TB").unwrap().to_size(), ByteSize::tb(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 GB").unwrap().to_size(), ByteSize::gb(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 MB").unwrap().to_size(), ByteSize::mb(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 KB").unwrap().to_size(), ByteSize::kb(12.3).to_size());

    assert_eq!(ByteSize::from_str("12.3 PiB").unwrap().to_size(), ByteSize::pib(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 TiB").unwrap().to_size(), ByteSize::tib(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 GiB").unwrap().to_size(), ByteSize::gib(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 MiB").unwrap().to_size(), ByteSize::mib(12.3).to_size());
    assert_eq!(ByteSize::from_str("12.3 KiB").unwrap().to_size(), ByteSize::kib(12.3).to_size());

    assert_eq!("12.3 PB",ByteSize::pb(12.3).to_pb().to_string())
}

#[test]
fn convert() {
    assert_eq!(ByteSize::from_str("12.3 PB").unwrap().to_tb().to_string(), ByteSize::pb(12.3).to_tb().to_string());
    // ...
}

#[test]
fn calculate(){
    let a=ByteSize::gb(2.0);
    let b=ByteSize::mb(1000.0);
    assert_eq!(add(a,b).unwrap().to_size(),ByteSize::gb(3.0).to_size())
}

} ```