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/transaction-payment/src/payment.rs

@@ -18,11 +18,11 @@
 /// ! Traits and default implementation for paying transaction fees.
 use crate::Config;
 
+use core::marker::PhantomData;
 use sp_runtime::{
 	traits::{DispatchInfoOf, PostDispatchInfoOf, Saturating, Zero},
 	transaction_validity::InvalidTransaction,
 };
-use sp_std::marker::PhantomData;
 
 use frame_support::{
 	traits::{Currency, ExistenceRequirement, Imbalance, OnUnbalanced, WithdrawReasons},