Remove unnecessary 'as' statement

We can just use `self`, no idea why the `self as io` is there.

src/util/bip158.rs

@@ -47,7 +47,7 @@
 
 use prelude::*;
 
-use io::{self as io, Cursor};
+use io::{self, Cursor};
 use core::fmt::{self, Display, Formatter};
 use core::cmp::{self, Ordering};