Simplify multiple use declarations in same module

[zombiezen]

(I'm a Rust noob, so this might not be a great idea.)

I found myself following this pattern in my learning project:

use std::io;
use std::io::BufRead;
use std::io::Read;

when it could have been written much more concisely as:

use std::io::{self, BufRead, Read};

It seems like if you have a bunch of use declarations in sequence, rustfmt could use some heuristics and condense them. I searched through the issue tracker and couldn't find anything like this.