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/frame/support/src/traits/error.rs

@@ -17,7 +17,7 @@
 
 //! Traits for describing and constraining pallet error types.
 use codec::{Compact, Decode, Encode};
-use sp_std::marker::PhantomData;
+use core::marker::PhantomData;
 
 /// Trait indicating that the implementing type is going to be included as a field in a variant of
 /// the `#[pallet::error]` enum type.