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/api/proc-macro/src/impl_runtime_apis.rs

@@ -243,7 +243,7 @@ fn generate_wasm_interface(impls: &[ItemImpl]) -> Result<TokenStream> {
 								&[0u8; 0]
 							} else {
 								unsafe {
-									#c::slice::from_raw_parts(input_data, input_len)
+									::core::slice::from_raw_parts(input_data, input_len)
 								}
 							};
 
@@ -345,7 +345,7 @@ fn generate_runtime_api_base_structures() -> Result<TokenStream> {
 					&self,
 					backend: &B,
 					parent_hash: Block::Hash,
-				) -> core::result::Result<
+				) -> ::core::result::Result<
 					#crate_::StorageChanges<Block>,
 				String
 					> where Self: Sized {