Add heaviest subtree utility functions (#10863)

Co-authored-by: Carl <carl@solana.com>

runtime/src/lib.rs

@@ -5,6 +5,7 @@ pub mod append_vec;
 pub mod bank;
 pub mod bank_client;
 pub mod bank_forks;
+pub mod bank_utils;
 mod blockhash_queue;
 pub mod bloom;
 pub mod builtin_programs;