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/examples/basic/src/weights.rs

@@ -42,7 +42,7 @@
 #![allow(unused_imports)]
 
 use frame_support::{traits::Get, weights::{Weight, constants::RocksDbWeight}};
-use sp_std::marker::PhantomData;
+use core::marker::PhantomData;
 
 /// Weight functions needed for pallet_example_basic.
 pub trait WeightInfo {