Fix unused imports

    warning: unused imports: `cmp`, `mem`, `slice`
       --> serde/src/lib.rs:171:26
        |
    171 |     pub use self::core::{cmp, iter, mem, num, ptr, slice, str};
        |                          ^^^        ^^^            ^^^^^
        |
        = note: `#[warn(unused_imports)]` on by default

serde/src/lib.rs

@@ -168,11 +168,14 @@ mod lib {
         pub use std::*;
     }
 
-    pub use self::core::{cmp, iter, mem, num, ptr, slice, str};
     pub use self::core::{f32, f64};
     pub use self::core::{i16, i32, i64, i8, isize};
+    pub use self::core::{iter, num, ptr, str};
     pub use self::core::{u16, u32, u64, u8, usize};
 
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    pub use self::core::{cmp, mem, slice};
+
     pub use self::core::cell::{Cell, RefCell};
     pub use self::core::clone::{self, Clone};
     pub use self::core::cmp::Reverse;