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/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::*;