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/state-machine/src/stats.rs

@@ -17,7 +17,7 @@
 
 //! Usage statistics for state db
 
-use sp_std::cell::RefCell;
+use core::cell::RefCell;
 #[cfg(feature = "std")]
 use std::time::{Duration, Instant};