sp-std -> core (#3199)

First in a series of PRs that reduces our use of sp-std with a view to
deprecating it.

This is just looking at /substrate and moving some of the references
from `sp-std` to `core`.
These particular changes should be uncontroversial.

Where macros are used `::core` should be used to remove any ambiguity.

part of https://github.com/paritytech/polkadot-sdk/issues/2101

substrate/frame/atomic-swap/src/lib.rs

@@ -43,6 +43,10 @@
 mod tests;
 
 use codec::{Decode, Encode};
+use core::{
+	marker::PhantomData,
+	ops::{Deref, DerefMut},
+};
 use frame_support::{
 	dispatch::DispatchResult,
 	pallet_prelude::MaxEncodedLen,
@@ -54,11 +58,6 @@ use frame_system::pallet_prelude::BlockNumberFor;
 use scale_info::TypeInfo;
 use sp_io::hashing::blake2_256;
 use sp_runtime::RuntimeDebug;
-use sp_std::{
-	marker::PhantomData,
-	ops::{Deref, DerefMut},
-	prelude::*,
-};
 
 /// Pending atomic swap operation.
 #[derive(Clone, Eq, PartialEq, RuntimeDebugNoBound, Encode, Decode, TypeInfo, MaxEncodedLen)]