Refactor WorkflowNode into AbstractWorkflowNode <- StatefulWorkflowNode.

workflow-runtime/src/commonMain/kotlin/com/squareup/workflow1/WorkflowInterceptor.kt

@@ -1,6 +1,7 @@
 package com.squareup.workflow1
 
 import com.squareup.workflow1.WorkflowInterceptor.RenderContextInterceptor
+import com.squareup.workflow1.WorkflowInterceptor.RuntimeUpdate
 import com.squareup.workflow1.WorkflowInterceptor.WorkflowSession
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Job
@@ -194,6 +195,8 @@ public interface WorkflowInterceptor {
   /**
    * Information about the session of a workflow in the runtime that a [WorkflowInterceptor] method
    * is intercepting.
+   *
+   * Implementations should override [toString] to call [WorkflowSession.workflowSessionToString].
    */
   public interface WorkflowSession {
     /** The [WorkflowIdentifier] that represents the type of this workflow. */
@@ -419,6 +422,16 @@ internal fun <P, S, O, R> WorkflowInterceptor.intercept(
   }
 }
 
+internal fun WorkflowSession.workflowSessionToString(): String {
+  val parentDescription = parent?.let { "WorkflowInstance(…)" }
+  return "WorkflowInstance(" +
+    "identifier=$identifier, " +
+    "renderKey=$renderKey, " +
+    "instanceId=$sessionId, " +
+    "parent=$parentDescription" +
+    ")"
+}
+
 private class InterceptedRenderContext<P, S, O>(
   private val baseRenderContext: BaseRenderContext<P, S, O>,
   private val interceptor: RenderContextInterceptor<P, S, O>