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/generic/header.rs

@@ -133,7 +133,7 @@ where
 impl<Number, Hash> Header<Number, Hash>
 where
 	Number: Member
-		+ sp_std::hash::Hash
+		+ core::hash::Hash
 		+ Copy
 		+ MaybeDisplay
 		+ AtLeast32BitUnsigned