uses structural sharing for stake-delegations hash-map (#23585)

StakeDelegations is using Arc to implement copy-on-write semantics:
https://github.com/solana-labs/solana/blob/58c0db970/runtime/src/stake_delegations.rs#L14-L16

However a single delegation change will still clone the entire hash-map,
resulting in excessive memory use as observed in:
https://github.com/solana-labs/solana/issues/23061#issuecomment-1063444072

This commit instead uses immutable hash-map implementing structural
sharing:
> which means that if two data structures are mostly copies of each
> other, most of the memory they take up will be shared between them.
https://docs.rs/im/latest/im/

runtime/src/lib.rs

@@ -52,7 +52,6 @@ pub mod snapshot_hash;
 pub mod snapshot_package;
 pub mod snapshot_utils;
 pub mod sorted_storages;
-pub mod stake_delegations;
 pub mod stake_history;
 pub mod stake_weighted_timestamp;
 pub mod stakes;