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/primitives/runtime/src/offchain/storage_lock.rs

@@ -66,9 +66,9 @@ use crate::{
 	traits::BlockNumberProvider,
 };
 use codec::{Codec, Decode, Encode};
+use core::fmt;
 use sp_core::offchain::{Duration, Timestamp};
 use sp_io::offchain;
-use sp_std::fmt;
 
 /// Default expiry duration for time based locks in milliseconds.
 const STORAGE_LOCK_DEFAULT_EXPIRY_DURATION: Duration = Duration::from_millis(20_000);