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/consensus/babe/src/digests.rs

@@ -22,9 +22,10 @@ use super::{
 	BabeEpochConfiguration, Randomness, Slot, BABE_ENGINE_ID,
 };
 
+#[cfg(not(feature = "std"))]
+use alloc::vec::Vec;
 use sp_core::sr25519::vrf::VrfSignature;
 use sp_runtime::{DigestItem, RuntimeDebug};
-use sp_std::vec::Vec;
 
 use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;

substrate/primitives/consensus/babe/src/lib.rs

@@ -20,15 +20,18 @@
 #![forbid(unsafe_code, missing_docs, unused_variables, unused_imports)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
+extern crate alloc;
+
 pub mod digests;
 pub mod inherents;
 
+#[cfg(not(feature = "std"))]
+use alloc::vec::Vec;
 use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 use sp_runtime::{traits::Header, ConsensusEngineId, RuntimeDebug};
-use sp_std::vec::Vec;
 
 use crate::digests::{NextConfigDescriptor, NextEpochDescriptor};