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/sassafras/src/mock.rs

@@ -99,7 +99,7 @@ pub fn new_test_ext_with_pairs(
 	pallet_sassafras::GenesisConfig::<Test> {
 		authorities: authorities.clone(),
 		epoch_config: TEST_EPOCH_CONFIGURATION,
-		_phantom: sp_std::marker::PhantomData,
+		_phantom: core::marker::PhantomData,
 	}
 	.assimilate_storage(&mut storage)
 	.unwrap();