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/authority-discovery/Cargo.toml

@@ -21,7 +21,6 @@ scale-info = { version = "2.10.0", default-features = false, features = ["derive
 sp-api = { path = "../api", default-features = false }
 sp-application-crypto = { path = "../application-crypto", default-features = false }
 sp-runtime = { path = "../runtime", default-features = false }
-sp-std = { path = "../std", default-features = false }
 
 [features]
 default = ["std"]
@@ -31,7 +30,6 @@ std = [
 	"sp-api/std",
 	"sp-application-crypto/std",
 	"sp-runtime/std",
-	"sp-std/std",
 ]
 serde = [
 	"scale-info/serde",

substrate/primitives/authority-discovery/src/lib.rs

@@ -19,7 +19,9 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use sp_std::vec::Vec;
+extern crate alloc;
+
+use alloc::vec::Vec;
 
 mod app {
 	use sp_application_crypto::{app_crypto, key_types::AUTHORITY_DISCOVERY, sr25519};