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(