Add trait for saturating arithmetic (#15812)

* Add SaturatingArithmetic trait

* Use Duration saturating arithmetic

* Use new macro to fix poh_config

sdk/src/lib.rs

@@ -8,6 +8,7 @@ pub use solana_program::*;
 
 pub mod account;
 pub mod account_utils;
+pub mod arithmetic;
 pub mod builtins;
 pub mod client;
 pub mod commitment_config;