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/democracy/src/conviction.rs

@@ -19,12 +19,12 @@
 
 use crate::types::Delegations;
 use codec::{Decode, Encode, MaxEncodedLen};
+use core::result::Result;
 use scale_info::TypeInfo;
 use sp_runtime::{
 	traits::{Bounded, CheckedDiv, CheckedMul, Zero},
 	RuntimeDebug,
 };
-use sp_std::{prelude::*, result::Result};
 
 /// A value denoting the strength of conviction of a vote.
 #[derive(

substrate/frame/democracy/src/migrations/v1.rs

@@ -54,7 +54,7 @@ pub mod v1 {
 	use super::*;
 
 	/// Migration for translating bare `Hash`es into `Bounded<Call>`s.
-	pub struct Migration<T>(sp_std::marker::PhantomData<T>);
+	pub struct Migration<T>(core::marker::PhantomData<T>);
 
 	impl<T: Config + frame_system::Config<Hash = H256>> OnRuntimeUpgrade for Migration<T> {
 		#[cfg(feature = "try-runtime")]

substrate/frame/democracy/src/vote_threshold.rs

@@ -19,11 +19,11 @@
 
 use crate::Tally;
 use codec::{Decode, Encode, MaxEncodedLen};
+use core::ops::{Add, Div, Mul, Rem};
 use scale_info::TypeInfo;
 #[cfg(feature = "std")]
 use serde::{Deserialize, Serialize};
 use sp_runtime::traits::{IntegerSquareRoot, Zero};
-use sp_std::ops::{Add, Div, Mul, Rem};
 
 /// A means of determining if a vote is past pass threshold.
 #[derive(