[PEx] Fix serialization issues found by P regression tests

Author: aman-goel

Src/PRuntimes/PExplicitRuntime/src/main/java/pexplicit/runtime/machine/events/PContinuation.java

@@ -8,13 +8,14 @@
 import pexplicit.values.PMessage;
 import pexplicit.values.PValue;
 
+import java.io.Serializable;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 
 @Getter
-public class PContinuation {
+public class PContinuation implements Serializable {
     private final Set<String> caseEvents;
     private final SerializableBiFunction<PMachine, PMessage> handleFun;
     @Getter

Src/PRuntimes/PExplicitRuntime/src/main/java/pexplicit/runtime/scheduler/choice/ScheduleChoice.java

@@ -20,7 +20,7 @@ public class ScheduleChoice extends Choice<PMachineId> {
     /**
      * Protocol state at the schedule step
      */
-    private StepState choiceState = null;
+    private transient StepState choiceState = null;
 
     /**
      * Constructor