auto merge of #12326 : bjz/rust/integer, r=alexcrichton

This is part of the effort to simplify `std::num`, as tracked in issue #10387.

src/test/bench/shootout-pidigits.rs

@@ -14,6 +14,7 @@ use std::from_str::FromStr;
 use std::num::One;
 use std::num::Zero;
 use std::num::FromPrimitive;
+use num::Integer;
 use num::bigint::BigInt;
 
 struct Context {