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/construct_runtime/expand/call.rs

@@ -178,7 +178,7 @@ pub fn expand_outer_dispatch(
 			type PostInfo = #scrate::dispatch::PostDispatchInfo;
 			fn dispatch(self, origin: RuntimeOrigin) -> #scrate::dispatch::DispatchResultWithPostInfo {
 				if !<Self::RuntimeOrigin as #scrate::traits::OriginTrait>::filter_call(&origin, &self) {
-					return #scrate::__private::sp_std::result::Result::Err(
+					return ::core::result::Result::Err(
 						#system_path::Error::<#runtime>::CallFiltered.into()
 					);
 				}

substrate/frame/support/procedural/src/construct_runtime/expand/outer_enums.rs

@@ -134,7 +134,6 @@ pub fn expand_outer_enum(
 			enum_ty,
 		));
 		enum_conversions.extend(expand_enum_conversion(
-			scrate,
 			pallet_decl,
 			&pallet_enum,
 			&enum_name_ident,
@@ -220,7 +219,6 @@ fn expand_enum_variant(
 }
 
 fn expand_enum_conversion(
-	scrate: &TokenStream,
 	pallet: &Pallet,
 	pallet_enum: &TokenStream,
 	enum_name_ident: &Ident,
@@ -247,7 +245,7 @@ fn expand_enum_conversion(
 		impl TryInto<#pallet_enum> for #enum_name_ident {
 			type Error = ();
 
-			fn try_into(self) -> #scrate::__private::sp_std::result::Result<#pallet_enum, Self::Error> {
+			fn try_into(self) -> ::core::result::Result<#pallet_enum, Self::Error> {
 				match self {
 					Self::#variant_name(evt) => Ok(evt),
 					_ => Err(()),