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/society/src/migrations.rs

@@ -29,7 +29,7 @@ const TARGET: &'static str = "runtime::society::migration";
 
 /// This migration moves all the state to v2 of Society.
 pub struct VersionUncheckedMigrateToV2<T: Config<I>, I: 'static, PastPayouts>(
-	sp_std::marker::PhantomData<(T, I, PastPayouts)>,
+	core::marker::PhantomData<(T, I, PastPayouts)>,
 );
 
 impl<

substrate/frame/society/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_society.
 pub trait WeightInfo {