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

runtime/src/accounts_index.rs

@@ -7,7 +7,7 @@ type ForkList<T> = Vec<(Fork, T)>;
 
 #[derive(Debug, Default)]
 pub struct AccountsIndex<T> {
-    pub account_maps: hashbrown::HashMap<Pubkey, RwLock<ForkList<T>>>,
+    pub account_maps: HashMap<Pubkey, RwLock<ForkList<T>>>,
 
     pub roots: HashSet<Fork>,