Tidy the DEX pallet

substrate/median/src/lexicographic.rs

@@ -1,4 +1,5 @@
 use core::cmp::Ord;
+use alloc::vec::Vec;
 
 use scale::FullCodec;
 

substrate/median/src/lib.rs

@@ -1,10 +1,14 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc = include_str!("../README.md")]
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
 #![deny(missing_docs)]
 
 use core::cmp::Ordering;
 
+extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
+
 use scale::{EncodeLike, FullCodec};
 use frame_support::storage::*;