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/procedural/src/pallet/expand/call.rs

@@ -304,7 +304,7 @@ pub fn expand_call(def: &mut Def) -> proc_macro2::TokenStream {
 			#[doc(hidden)]
 			#[codec(skip)]
 			__Ignore(
-				#frame_support::__private::sp_std::marker::PhantomData<(#type_use_gen,)>,
+				::core::marker::PhantomData<(#type_use_gen,)>,
 				#frame_support::Never,
 			),
 			#(

substrate/frame/support/procedural/src/pallet/expand/event.rs

@@ -87,7 +87,7 @@ pub fn expand_event(def: &mut Def) -> proc_macro2::TokenStream {
 			#[doc(hidden)]
 			#[codec(skip)]
 			__Ignore(
-				#frame_support::__private::sp_std::marker::PhantomData<(#event_use_gen)>,
+				::core::marker::PhantomData<(#event_use_gen)>,
 				#frame_support::Never,
 			)
 		);

substrate/frame/support/procedural/src/pallet/parse/composite.rs

@@ -171,7 +171,7 @@ impl CompositeDef {
 				#[doc(hidden)]
 				#[codec(skip)]
 				__Ignore(
-					#scrate::__private::sp_std::marker::PhantomData<I>,
+					::core::marker::PhantomData<I>,
 				)
 			});
 		}