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/pow/Cargo.toml

@@ -20,7 +20,6 @@ codec = { package = "parity-scale-codec", version = "3.6.1", default-features =
 sp-api = { path = "../../api", default-features = false }
 sp-core = { path = "../../core", default-features = false }
 sp-runtime = { path = "../../runtime", default-features = false }
-sp-std = { path = "../../std", default-features = false }
 
 [features]
 default = ["std"]
@@ -29,5 +28,4 @@ std = [
 	"sp-api/std",
 	"sp-core/std",
 	"sp-runtime/std",
-	"sp-std/std",
 ]

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

@@ -19,9 +19,12 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
+extern crate alloc;
+
+#[cfg(not(feature = "std"))]
+use alloc::vec::Vec;
 use codec::Decode;
 use sp_runtime::ConsensusEngineId;
-use sp_std::vec::Vec;
 
 /// The `ConsensusEngineId` of PoW.
 pub const POW_ENGINE_ID: ConsensusEngineId = [b'p', b'o', b'w', b'_'];