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/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'_'];