Introduce type alias Ancestors (#9699)

* Introduce type alias AncestorList

* Rename AncestorList => Ancestors

accounts-bench/src/main.rs

@@ -1,9 +1,11 @@
 use clap::{value_t, App, Arg};
 use rayon::prelude::*;
 use solana_measure::measure::Measure;
-use solana_runtime::accounts::{create_test_accounts, update_accounts, Accounts};
+use solana_runtime::{
+    accounts::{create_test_accounts, update_accounts, Accounts},
+    accounts_index::Ancestors,
+};
 use solana_sdk::pubkey::Pubkey;
-use std::collections::HashMap;
 use std::fs;
 use std::path::PathBuf;
 
@@ -76,7 +78,7 @@ fn main() {
         num_slots,
         create_time
     );
-    let mut ancestors: HashMap<u64, usize> = vec![(0, 0)].into_iter().collect();
+    let mut ancestors: Ancestors = vec![(0, 0)].into_iter().collect();
     for i in 1..num_slots {
         ancestors.insert(i as u64, i - 1);
         accounts.add_root(i as u64);