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/sassafras/src/digests.rs

@@ -25,8 +25,9 @@ use crate::{
 use scale_codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
 
+#[cfg(not(feature = "std"))]
+use alloc::vec::Vec;
 use sp_runtime::{DigestItem, RuntimeDebug};
-use sp_std::vec::Vec;
 
 /// Epoch slot claim digest entry.
 ///