Implement the FromStr trait for ByteSize

[jRimbault]

A use case I came around when I wanted the user to provide a ByteSize in my program CLI arguments and using a argument parser like structopt or argh or gumdrop.

Allows parsing a string directly into a ByteSize:

let size = "128GB".parse::<ByteSize>().unwrap();

I'm bad at writing negative test cases though.

Short example of a binary using structopt taking a size as a command line argument:

use bytesize::ByteSize;
use structopt::StructOpt;

#[derive(Debug, StructOpt)]
struct Args {
    size: ByteSize,
}

fn main() {
    println!("{}", Args::from_args().size);
}

PS: the two 'static lifetime markers I removed because of clippy's suggestion

[orhun]

In case someone is looking for something like this: https://crates.io/crates/byte-unit

e.g. https://docs.rs/byte-unit/4.0.12/byte_unit/struct.Byte.html#method.from_str

[hyunsik]

Sorry for a late review. The patch looks straightforward and good to me.