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/benches/bloom.rs

@@ -6,8 +6,7 @@ use fnv::FnvHasher;
 use solana_runtime::bloom::{Bloom, BloomHashIndex};
 use solana_sdk::hash::{hash, Hash};
 use solana_sdk::signature::Signature;
-//use std::collections::HashSet;
-use hashbrown::HashSet;
+use std::collections::HashSet;
 use std::hash::Hasher;
 use test::Bencher;