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/alliance/src/benchmarking.rs

@@ -19,13 +19,12 @@
 
 #![cfg(feature = "runtime-benchmarks")]
 
-use sp_runtime::traits::{Bounded, Hash, StaticLookup};
-use sp_std::{
+use core::{
 	cmp,
 	convert::{TryFrom, TryInto},
 	mem::size_of,
-	prelude::*,
 };
+use sp_runtime::traits::{Bounded, Hash, StaticLookup};
 
 use frame_benchmarking::{account, impl_benchmark_test_suite, v2::*, BenchmarkError};
 use frame_support::traits::{EnsureOrigin, Get, UnfilteredDispatchable};