Access to special accounts with dynamically-updated data.
Sysvars are special accounts that contain dynamically-updated data about the
network cluster, the blockchain history, and the executing transaction. Each
sysvar is defined in its own crate. The clock, epoch_schedule,
instructions, and rent sysvars are most useful to on-chain programs.
All sysvar accounts are owned by the account identified by solana_sysvar::ID.
For more details see the Solana documentation on sysvars.