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/safe-mode/src/lib.rs

@@ -79,6 +79,7 @@ pub mod mock;
 mod tests;
 pub mod weights;
 
+use core::convert::TryInto;
 use frame_support::{
 	defensive_assert,
 	pallet_prelude::*,
@@ -96,7 +97,6 @@ use frame_support::{
 use frame_system::pallet_prelude::*;
 use sp_arithmetic::traits::Zero;
 use sp_runtime::traits::Saturating;
-use sp_std::{convert::TryInto, prelude::*};
 
 pub use pallet::*;
 pub use weights::*;