Add program_ids() methods

Added CompiledInstruction::program_id() so that we don't need to pass
around instruction indexes just for Message::program_id().

Also added Message.program_ids() that returns a slice so that we
can move those pubkeys into Message::account_keys.

sdk/src/instruction.rs

@@ -129,4 +129,8 @@ impl CompiledInstruction {
             accounts,
         }
     }
+
+    pub fn program_id<'a>(&self, program_ids: &'a [Pubkey]) -> &'a Pubkey {
+        &program_ids[self.program_ids_index as usize]
+    }
 }