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/inherents/src/lib.rs

@@ -162,9 +162,11 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(missing_docs)]
 
+extern crate alloc;
+
 use codec::{Decode, Encode};
 
-use sp_std::{
+use alloc::{
 	collections::btree_map::{BTreeMap, Entry, IntoIter},
 	vec::Vec,
 };