sp-std -> core (#3199)

First in a series of PRs that reduces our use of sp-std with a view to
deprecating it.

This is just looking at /substrate and moving some of the references
from `sp-std` to `core`.
These particular changes should be uncontroversial.

Where macros are used `::core` should be used to remove any ambiguity.

part of https://github.com/paritytech/polkadot-sdk/issues/2101

substrate/primitives/runtime-interface/src/util.rs

@@ -21,7 +21,7 @@
 pub fn pack_ptr_and_len(ptr: u32, len: u32) -> u64 {
 	// The static assertions from above are changed into a runtime check.
 	#[cfg(all(not(feature = "std"), feature = "disable_target_static_assertions"))]
-	assert_eq!(4, sp_std::mem::size_of::<usize>());
+	assert_eq!(4, core::mem::size_of::<usize>());
 
 	(u64::from(len) << 32) | u64::from(ptr)
 }
@@ -34,7 +34,7 @@ pub fn pack_ptr_and_len(ptr: u32, len: u32) -> u64 {
 pub fn unpack_ptr_and_len(val: u64) -> (u32, u32) {
 	// The static assertions from above are changed into a runtime check.
 	#[cfg(all(not(feature = "std"), feature = "disable_target_static_assertions"))]
-	assert_eq!(4, sp_std::mem::size_of::<usize>());
+	assert_eq!(4, core::mem::size_of::<usize>());
 
 	let ptr = (val & (!0u32 as u64)) as u32;
 	let len = (val >> 32) as u32;

substrate/primitives/runtime-interface/src/wasm.rs

@@ -19,7 +19,7 @@
 
 use crate::RIType;
 
-use sp_std::cell::Cell;
+use core::cell::Cell;
 
 /// Something that can be created from a ffi value.
 ///