A type to hold data for the [`SlotHashes` sysvar][sv].
[sv]: https://docs.solanalabs.com/runtime/sysvars#slothashes
The sysvar ID is declared in [`solana_program::sysvar::slot_hashes`].
[`solana_program::sysvar::slot_hashes`]: https://docs.rs/solana-program/latest/solana_program/sysvar/slot_hashes/index.html