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/collective/src/benchmarking.rs

@@ -20,8 +20,8 @@
 use super::*;
 use crate::Pallet as Collective;
 
+use core::mem::size_of;
 use sp_runtime::traits::Bounded;
-use sp_std::mem::size_of;
 
 use frame_benchmarking::v1::{account, benchmarks_instance_pallet, whitelisted_caller};
 use frame_system::{