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/examples/basic/src/lib.rs

@@ -54,6 +54,7 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use codec::{Decode, Encode};
+use core::marker::PhantomData;
 use frame_support::{
 	dispatch::{ClassifyDispatch, DispatchClass, DispatchResult, Pays, PaysFee, WeighData},
 	traits::IsSubType,
@@ -68,7 +69,7 @@ use sp_runtime::{
 		InvalidTransaction, TransactionValidity, TransactionValidityError, ValidTransaction,
 	},
 };
-use sp_std::{marker::PhantomData, prelude::*};
+use sp_std::vec::Vec;
 
 // Re-export pallet items so that they can be accessed from the crate namespace.
 pub use pallet::*;
@@ -485,8 +486,8 @@ impl<T: Config> Pallet<T> {
 #[scale_info(skip_type_params(T))]
 pub struct WatchDummy<T: Config + Send + Sync>(PhantomData<T>);
 
-impl<T: Config + Send + Sync> sp_std::fmt::Debug for WatchDummy<T> {
-	fn fmt(&self, f: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {
+impl<T: Config + Send + Sync> core::fmt::Debug for WatchDummy<T> {
+	fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
 		write!(f, "WatchDummy")
 	}
 }
@@ -501,7 +502,7 @@ where
 	type AdditionalSigned = ();
 	type Pre = ();
 
-	fn additional_signed(&self) -> sp_std::result::Result<(), TransactionValidityError> {
+	fn additional_signed(&self) -> core::result::Result<(), TransactionValidityError> {
 		Ok(())
 	}
 

substrate/frame/examples/basic/src/weights.rs

@@ -42,7 +42,7 @@
 #![allow(unused_imports)]
 
 use frame_support::{traits::Get, weights::{Weight, constants::RocksDbWeight}};
-use sp_std::marker::PhantomData;
+use core::marker::PhantomData;
 
 /// Weight functions needed for pallet_example_basic.
 pub trait WeightInfo {