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/referenda/src/types.rs

@@ -19,6 +19,7 @@
 
 use super::*;
 use codec::{Decode, Encode, EncodeLike, MaxEncodedLen};
+use core::fmt::Debug;
 use frame_support::{
 	traits::{schedule::v3::Anon, Bounded},
 	Parameter,
@@ -26,7 +27,6 @@ use frame_support::{
 use scale_info::TypeInfo;
 use sp_arithmetic::{Rounding::*, SignedRounding::*};
 use sp_runtime::{FixedI64, PerThing, RuntimeDebug};
-use sp_std::fmt::Debug;
 
 pub type BalanceOf<T, I = ()> =
 	<<T as Config<I>>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;