Boot hashbrown (#5505)

As of Rust 1.36.0, hashbrown now implements the HashMap in std (which
implements HashSet).

https://blog.rust-lang.org/2019/07/04/Rust-1.36.0.html#a-new-hashmapk,-v%3E-implementation

local_cluster/Cargo.toml

@@ -21,7 +21,6 @@ solana-storage-program = { path = "../programs/storage_program", version = "0.18
 solana-vote-api = { path = "../programs/vote_api", version = "0.18.0-pre1" }
 
 [dev-dependencies]
-hashbrown = "0.5.0"
 serial_test = "0.2.0"
 serial_test_derive = "0.2.0"
 

local_cluster/tests/local_cluster.rs

@@ -1,6 +1,5 @@
 extern crate solana;
 
-use hashbrown::HashSet;
 use log::*;
 use serial_test_derive::serial;
 use solana::blocktree::Blocktree;
@@ -14,6 +13,7 @@ use solana_runtime::epoch_schedule::{EpochSchedule, MINIMUM_SLOTS_PER_EPOCH};
 use solana_sdk::client::SyncClient;
 use solana_sdk::poh_config::PohConfig;
 use solana_sdk::timing;
+use std::collections::HashSet;
 use std::thread::sleep;
 use std::time::Duration;