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/uniques/src/migration.rs

@@ -17,8 +17,8 @@
 
 //! Various pieces of common functionality.
 use super::*;
+use core::marker::PhantomData;
 use frame_support::traits::{Get, OnRuntimeUpgrade};
-use sp_std::marker::PhantomData;
 
 mod v1 {
 	use super::*;