sp-std removal from substrate/primitives (#3274)

This PR removes sp-std crate from substrate/primitives sub-directories.

For now crates that have `pub use` of sp-std or export macros that would
necessitate users of the macros to `extern crate alloc` have been
excluded from this PR.

There should be no breaking changes in this PR.

---------

Co-authored-by: Koute <koute@users.noreply.github.com>

substrate/primitives/npos-elections/src/reduce.rs

@@ -51,11 +51,12 @@ use crate::{
 	node::{Node, NodeId, NodeRef, NodeRole},
 	ExtendedBalance, IdentifierT, StakedAssignment,
 };
-use sp_arithmetic::traits::{Bounded, Zero};
-use sp_std::{
+use alloc::{
 	collections::btree_map::{BTreeMap, Entry::*},
-	prelude::*,
+	vec,
+	vec::Vec,
 };
+use sp_arithmetic::traits::{Bounded, Zero};
 
 /// Map type used for reduce_4. Can be easily swapped with HashMap.
 type Map<A> = BTreeMap<(A, A), A>;