Make HexDisplay useable in no_std (#6883)

Actually I use this quite often when debugging some WASM bugs and there
is no harm in enabling it by default. Before I just always copied it
everytime I needed it.

substrate/primitives/core/src/lib.rs

@@ -52,7 +52,6 @@ pub use impl_serde::serialize as bytes;
 pub mod hashing;
 #[cfg(feature = "full_crypto")]
 pub use hashing::{blake2_128, blake2_256, twox_64, twox_128, twox_256, keccak_256};
-#[cfg(feature = "std")]
 pub mod hexdisplay;
 pub mod crypto;