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/contracts/src/gas.rs

@@ -16,6 +16,7 @@
 // limitations under the License.
 
 use crate::{exec::ExecError, Config, Error};
+use core::marker::PhantomData;
 use frame_support::{
 	dispatch::{DispatchErrorWithPostInfo, DispatchResultWithPostInfo, PostDispatchInfo},
 	weights::Weight,
@@ -23,7 +24,6 @@ use frame_support::{
 };
 use sp_core::Get;
 use sp_runtime::{traits::Zero, DispatchError};
-use sp_std::marker::PhantomData;
 
 #[cfg(test)]
 use std::{any::Any, fmt::Debug};

substrate/frame/contracts/src/migration/v10.rs

@@ -25,7 +25,10 @@ use crate::{
 	CodeHash, Config, Pallet, TrieId, Weight, LOG_TARGET,
 };
 use codec::{Decode, Encode};
-use core::cmp::{max, min};
+use core::{
+	cmp::{max, min},
+	ops::Deref,
+};
 use frame_support::{
 	pallet_prelude::*,
 	storage_alias,
@@ -42,7 +45,7 @@ use sp_runtime::{
 	traits::{Hash, TrailingZeroInput, Zero},
 	Perbill, Saturating,
 };
-use sp_std::{ops::Deref, prelude::*};
+use sp_std::prelude::*;
 
 mod old {
 	use super::*;

substrate/frame/contracts/src/schedule.rs

@@ -21,11 +21,11 @@
 use crate::{weights::WeightInfo, Config};
 
 use codec::{Decode, Encode};
+use core::marker::PhantomData;
 use frame_support::{weights::Weight, DefaultNoBound};
 use scale_info::TypeInfo;
 #[cfg(feature = "std")]
 use serde::{Deserialize, Serialize};
-use sp_std::marker::PhantomData;
 
 /// Definition of the cost schedule and other parameterizations for the wasm vm.
 ///