Prefer using core to std for the same reason as previous commit

src/groupbylazy.rs

@@ -1,7 +1,7 @@
-use alloc::vec::{self, Vec};
-use std::cell::{Cell, RefCell};
-use std::ops::Deref;
 use alloc::rc::Rc;
+use alloc::vec::{self, Vec};
+use core::cell::{Cell, RefCell};
+use core::ops::Deref;
 
 /// A trait to unify `FnMut` for `ChunkBy` with the chunk key in `IntoChunks`
 trait KeyFunction<A> {