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.
 ///