Accept dynamically sized Read, Write, and Serialize implementations

[dtolnay]

See #95 (comment).

Sure! Accepting a ?Sized type parameter for io::Read and io::Write means that it will work with &mut io::Read and &mut io::Write trait objects which are dynamically sized.

use std::fs::File;
use std::io::Read;

fn main() {
    let mut data = File::open("important.bin").unwrap();
    let trait_object: &mut Read = &mut data;
    deserialize_from(trait_object); // requires ?Sized
}

fn deserialize_from<R>(_: &mut R) where R: Read {}

And accepting a ?Sized type parameter for &T means that it can work with dynamically sized implementations of Serialize, which includes str and [T] among others.

fn main() {
    serialize("&str"); // requires ?Sized
}

trait Serialize {}
impl Serialize for str {}

fn serialize<T>(_: &T) where T: Serialize {}