Make Saveable serializable

It was marked with `@Contextual`, but a serializer was never being passed in, therefore it crashes as it can't find a serializer. The class is trivial to serialize, so we can just annotate it with `@Serializable` for a serialization adapter to be generated for us.

redwood-treehouse/src/commonMain/kotlin/app/cash/redwood/treehouse/StateSnapshot.kt

@@ -18,7 +18,6 @@ package app.cash.redwood.treehouse
 import androidx.compose.runtime.MutableState
 import androidx.compose.runtime.mutableStateOf
 import kotlin.jvm.JvmInline
-import kotlinx.serialization.Contextual
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonArray
 import kotlinx.serialization.json.JsonElement
@@ -29,7 +28,7 @@ import kotlinx.serialization.json.intOrNull
 
 @Serializable
 public class StateSnapshot(
-  public val content: Map<String, List<@Contextual Saveable>>,
+  public val content: Map<String, List<Saveable>>,
 ) {
   public fun toValuesMap(): Map<String, List<Any?>> {
     return content.mapValues { entry ->
@@ -89,6 +88,7 @@ private fun JsonElement?.fromJsonElement(): Any {
   }
 }
 
+@Serializable
 public data class Saveable(
   val isMutableState: Boolean,
   val value: JsonElement,