Fix Rust beta compiler errors 1.78.0-beta.1 (#5439)

* remove redundant imports

* fix test

* contains key

* fmt

* Merge branch 'unstable' into fix-beta-compiler

slasher/src/array.rs

@@ -7,9 +7,7 @@ use flate2::bufread::{ZlibDecoder, ZlibEncoder};
 use serde::{Deserialize, Serialize};
 use std::borrow::Borrow;
 use std::collections::{btree_map::Entry, BTreeMap, HashSet};
-use std::convert::TryFrom;
 use std::io::Read;
-use std::iter::Extend;
 use std::sync::Arc;
 use types::{AttesterSlashing, Epoch, EthSpec, IndexedAttestation};