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/salary/src/lib.rs

@@ -21,10 +21,10 @@
 #![recursion_limit = "128"]
 
 use codec::{Decode, Encode, MaxEncodedLen};
+use core::marker::PhantomData;
 use scale_info::TypeInfo;
 use sp_arithmetic::traits::{Saturating, Zero};
 use sp_runtime::{Perbill, RuntimeDebug};
-use sp_std::{marker::PhantomData, prelude::*};
 
 use frame_support::{
 	defensive,

substrate/frame/salary/src/tests/unit.rs

@@ -19,6 +19,7 @@
 
 use std::collections::BTreeMap;
 
+use core::cell::RefCell;
 use frame_support::{
 	assert_noop, assert_ok, derive_impl,
 	pallet_prelude::Weight,
@@ -26,7 +27,6 @@ use frame_support::{
 	traits::{tokens::ConvertRank, ConstU64},
 };
 use sp_runtime::{traits::Identity, BuildStorage, DispatchResult};
-use sp_std::cell::RefCell;
 
 use crate as pallet_salary;
 use crate::*;