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-multi-phase/src/benchmarking.rs

@@ -496,7 +496,7 @@ frame_benchmarking::benchmarks! {
 				let (_, stake, _) = voters[*idx];
 				stake
 			}).unwrap_or_default();
-			sp_std::cmp::Reverse(stake)
+			core::cmp::Reverse(stake)
 		});
 
 		let mut index_assignments = assignments