Optimize Transaction/Instruction serialization with custom routine (#2515)

* Optimize transaction serialization with custom routine to reduce the serialized size.

* Update serialized_size to accept self as parameter

* Optimize serialize / deserialize operations

sdk/src/lib.rs

@@ -12,6 +12,7 @@ pub mod native_program;
 pub mod packet;
 pub mod payment_plan;
 pub mod pubkey;
+pub mod shortvec;
 pub mod signature;
 pub mod storage_program;
 pub mod system_instruction;