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/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.
 ///