Add default impls for ContinuationFrame's depth and subIotas to reduce addon breakage

Author: vgskye

Common/src/main/java/at/petrak/hexcasting/api/casting/eval/vm/ContinuationFrame.kt

@@ -46,8 +46,8 @@ interface ContinuationFrame {
      * Return the number of iotas contained inside this frame, used for determining whether it is valid to serialise.
      */
     fun size(): Int
-    fun depth(): Int
-    fun subIotas(): Iterable<Iota>?
+    fun depth(): Int = 0
+    fun subIotas(): Iterable<Iota>? = null
 
     val type: Type<*>