Rent tangential stuff (#5910)

* rename rent.rs to rent_calculator.rs

* add rent sysvar

* integrate rent_calculator with bank

* rent_calculator integration with genesis

* add test for rent sysvar

sdk/src/sysvar/mod.rs

@@ -4,6 +4,7 @@ use crate::pubkey::Pubkey;
 
 pub mod clock;
 pub mod fees;
+pub mod rent;
 pub mod rewards;
 pub mod slot_hashes;
 pub mod stake_history;