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/asset-conversion/src/mock.rs

@@ -19,6 +19,7 @@
 
 use super::*;
 use crate as pallet_asset_conversion;
+use core::default::Default;
 use frame_support::{
 	construct_runtime, derive_impl,
 	instances::{Instance1, Instance2},
@@ -39,7 +40,6 @@ use sp_runtime::{
 	traits::{AccountIdConversion, BlakeTwo256, IdentityLookup},
 	BuildStorage,
 };
-use sp_std::default::Default;
 
 type Block = frame_system::mocking::MockBlock<Test>;
 

substrate/frame/asset-conversion/src/types.rs

@@ -17,8 +17,8 @@
 
 use super::*;
 use codec::{Decode, Encode, MaxEncodedLen};
+use core::marker::PhantomData;
 use scale_info::TypeInfo;
-use sp_std::marker::PhantomData;
 
 /// Represents a swap path with associated asset amounts indicating how much of the asset needs to
 /// be deposited to get the following asset's amount withdrawn (this is inclusive of fees).