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/nfts/src/macros.rs

@@ -42,7 +42,7 @@ macro_rules! impl_codec_bitflags {
 		impl Decode for $wrapper {
 			fn decode<I: codec::Input>(
 				input: &mut I,
-			) -> sp_std::result::Result<Self, codec::Error> {
+			) -> ::core::result::Result<Self, codec::Error> {
 				let field = <$size>::decode(input)?;
 				Ok(Self(BitFlags::from_bits(field as $size).map_err(|_| "invalid value")?))
 			}

substrate/frame/nfts/src/migration.rs

@@ -51,7 +51,7 @@ pub mod v1 {
 	}
 
 	/// A migration utility to update the storage version from v0 to v1 for the pallet.
-	pub struct MigrateToV1<T>(sp_std::marker::PhantomData<T>);
+	pub struct MigrateToV1<T>(core::marker::PhantomData<T>);
 	impl<T: Config> OnRuntimeUpgrade for MigrateToV1<T> {
 		fn on_runtime_upgrade() -> Weight {
 			let current_version = Pallet::<T>::current_storage_version();