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/election-provider-support/src/tests.rs

@@ -29,7 +29,7 @@ mod solution_type {
 	// these need to come from the same dev-dependency `frame-election-provider-support`, not from
 	// the crate.
 	use crate::{generate_solution_type, Assignment, Error as NposError, NposSolution};
-	use sp_std::fmt::Debug;
+	use core::fmt::Debug;
 
 	#[allow(dead_code)]
 	mod __private {

substrate/frame/election-provider-support/src/weights.rs

@@ -41,7 +41,7 @@
 #![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_election_provider_support_benchmarking.
 pub trait WeightInfo {