Resolve redundant import warning

    warning: the item `Ord` is imported redundantly
       --> src/atom.rs:1:16
        |
    1   | use std::cmp::{Ord, Ordering, PartialOrd};
        |                ^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:125:13
        |
    125 |     pub use super::v1::*;
        |             --------- the item `Ord` is already defined here
        |
        = note: `#[warn(unused_imports)]` on by default

    warning: the item `PartialOrd` is imported redundantly
       --> src/atom.rs:1:31
        |
    1   | use std::cmp::{Ord, Ordering, PartialOrd};
        |                               ^^^^^^^^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:125:13
        |
    125 |     pub use super::v1::*;
        |             --------- the item `PartialOrd` is already defined here

src/atom.rs

@@ -1,4 +1,4 @@
-use std::cmp::{Ord, Ordering, PartialOrd};
+use std::cmp::Ordering;
 use std::str;
 
 #[derive(Copy, Clone, Eq, PartialEq)]