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/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.
 ///