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/im-online/src/migration.rs

@@ -56,7 +56,7 @@ pub mod v1 {
 	use super::*;
 
 	/// Simple migration that replaces `ReceivedHeartbeats` values with `true`.
-	pub struct Migration<T>(sp_std::marker::PhantomData<T>);
+	pub struct Migration<T>(core::marker::PhantomData<T>);
 
 	impl<T: Config> OnRuntimeUpgrade for Migration<T> {
 		#[cfg(feature = "try-runtime")]