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/state-machine/src/error.rs

@@ -16,7 +16,7 @@
 // limitations under the License.
 
 /// State Machine Errors
-use sp_std::fmt;
+use core::fmt;
 
 /// State Machine Error bound.
 ///

substrate/primitives/state-machine/src/in_memory_backend.rs

@@ -79,7 +79,7 @@ where
 
 	/// Apply the given transaction to this backend and set the root to the given value.
 	pub fn apply_transaction(&mut self, root: H::Out, transaction: PrefixedMemoryDB<H>) {
-		let mut storage = sp_std::mem::take(self).into_storage();
+		let mut storage = core::mem::take(self).into_storage();
 
 		storage.consolidate(transaction);
 		*self = TrieBackendBuilder::new(storage, root).build();

substrate/primitives/state-machine/src/stats.rs

@@ -17,7 +17,7 @@
 
 //! Usage statistics for state db
 
-use sp_std::cell::RefCell;
+use core::cell::RefCell;
 #[cfg(feature = "std")]
 use std::time::{Duration, Instant};