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/tests/mod.rs

@@ -171,7 +171,7 @@ pub mod frame_system {
 		pub data: Vec<(u32, u64)>,
 		pub test_config: Vec<(u32, u32, u64)>,
 		#[serde(skip)]
-		pub _config: sp_std::marker::PhantomData<T>,
+		pub _config: core::marker::PhantomData<T>,
 	}
 
 	impl<T: Config> Default for GenesisConfig<T> {