Implement the Write trait ?

Hi,

I've used seahash in the past, and yesterday I was reading this and I thought it would be "neat" to also have the Write trait implemented for the SeaHasher.

I'm copying the example I linked:

use seahash::SeaHasher;
use std::{fs, io};

let mut file = fs::File::open(&path)?;
let mut hasher = SeaHasher::new();
let n = io::copy(&mut file, &mut hasher)?;
let hash = hasher.finish();

I think it would require very little additional code to do:

use core::hash::Hasher;

#[cfg(feature = "std")]
impl std::io::Write for SeaHasher {
    fn write(&mut self, buf: &[u8]) -> st::io::Result<usize> {
        Hasher::write(self, buf);
        Ok(buf.len())
    }
    fn flush(&mut self) -> std::io::Result<()> {
        Ok(())
    }
}

Just an idea. But when I wanted to hash a whole file I ended writing almost the same code as the canonical io::copy. It can be placed behind an "std" feature flag.