Update hex-literal version, simplify imports and remove unused dependencies (#2371)

substrate/core/sr-primitives/src/lib.rs

@@ -33,9 +33,6 @@ use rstd::prelude::*;
 use substrate_primitives::{crypto, ed25519, sr25519, hash::{H256, H512}};
 use codec::{Encode, Decode};
 
-#[cfg(feature = "std")]
-use substrate_primitives::hexdisplay::ascii_format;
-
 #[cfg(feature = "std")]
 pub mod testing;