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/staking/src/election_size_tracker.rs

@@ -84,7 +84,7 @@ use frame_election_provider_support::{
 pub struct StaticTracker<DataProvider> {
 	pub size: usize,
 	pub counter: usize,
-	_marker: sp_std::marker::PhantomData<DataProvider>,
+	_marker: core::marker::PhantomData<DataProvider>,
 }
 
 impl<DataProvider> Default for StaticTracker<DataProvider> {