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/core-fellowship/src/tests/unit.rs

@@ -19,6 +19,7 @@
 
 use std::collections::BTreeMap;
 
+use core::cell::RefCell;
 use frame_support::{
 	assert_noop, assert_ok, derive_impl, ord_parameter_types,
 	pallet_prelude::Weight,
@@ -27,7 +28,6 @@ use frame_support::{
 };
 use frame_system::EnsureSignedBy;
 use sp_runtime::{traits::TryMorphInto, BuildStorage, DispatchError, DispatchResult};
-use sp_std::cell::RefCell;
 
 use crate as pallet_core_fellowship;
 use crate::*;