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/consensus/babe/src/inherents.rs

@@ -17,8 +17,8 @@
 
 //! Inherents for BABE
 
+use core::result::Result;
 use sp_inherents::{Error, InherentData, InherentIdentifier};
-use sp_std::result::Result;
 
 /// The BABE inherent identifier.
 pub const INHERENT_IDENTIFIER: InherentIdentifier = *b"babeslot";
@@ -75,7 +75,7 @@ impl InherentDataProvider {
 }
 
 #[cfg(feature = "std")]
-impl sp_std::ops::Deref for InherentDataProvider {
+impl core::ops::Deref for InherentDataProvider {
 	type Target = InherentType;
 
 	fn deref(&self) -> &Self::Target {