Optimize central unwrap implementations.

The default implementation of `Unwrappable.unwrap` creates a `Sequence` that is almost never necessary (but it is at least fail-safe so I think we should keep it). Here we provide improved implementations for `Wrapper` and `BackStackScreen`.

Author: rjrjr

workflow-ui/core-common/src/main/java/com/squareup/workflow1/ui/Container.kt

@@ -87,6 +87,8 @@ public interface Container<CategoryT, out C : CategoryT> : Composite<C> {
 public interface Wrapper<BaseT : Any, out C : BaseT> : Container<BaseT, C>, Compatible {
   public val content: C
 
+  override val unwrapped: Any get() = content
+
   /**
    * Default implementation makes this [Wrapper] compatible with others of the same type,
    * and which wrap compatible [content].

workflow-ui/core-common/src/main/java/com/squareup/workflow1/ui/navigation/BackStackScreen.kt

@@ -48,6 +48,8 @@ public class BackStackScreen<out StackedT : Screen> internal constructor(
 
   override val compatibilityKey: String = keyFor(this, name)
 
+  override val unwrapped: Any get() = top
+
   override fun asSequence(): Sequence<StackedT> = frames.asSequence()
 
   /**

workflow-ui/core-common/src/main/java/com/squareup/workflow1/ui/navigation/BodyAndOverlaysScreen.kt

@@ -79,6 +79,8 @@ public class BodyAndOverlaysScreen<B : Screen, O : Overlay>(
 ) : Screen, Compatible, Composite<Any> {
   override val compatibilityKey: String = keyFor(this, name)
 
+  override val unwrapped: Any = overlays.lastOrNull() ?: body
+
   override fun asSequence(): Sequence<Any> = sequenceOf(body) + overlays.asSequence()
 
   public fun <S : Screen> mapBody(transform: (B) -> S): BodyAndOverlaysScreen<S, O> {