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/aura/src/inherents.rs

@@ -69,7 +69,7 @@ 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 {