Override toString function in stdlib delegates

Co-authored-by: ilya-g <ilya.gorbunov@jetbrains.com>

libraries/stdlib/src/kotlin/properties/Delegates.kt

@@ -65,5 +65,8 @@ private class NotNullVar<T : Any>() : ReadWriteProperty<Any?, T> {
     public override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
         this.value = value
     }
+
+    public override fun toString(): String =
+        "NotNullProperty(${if (value != null) "value=$value" else "value not initialized yet"})"
 }
 

libraries/stdlib/src/kotlin/properties/ObservableProperty.kt

@@ -40,4 +40,6 @@ public abstract class ObservableProperty<V>(initialValue: V) : ReadWriteProperty
         this.value = value
         afterChange(property, oldValue, value)
     }
+
+    override fun toString(): String = "ObservableProperty(value=$value)"
 }