Added 'depth' counter to stack frames. The idea is two-fold:

1. You can limit runaway recursion by binding a symbol *max_stack_depth* in the environment
2. You can limit the number of backtrace frames printed.

However, neither of these have been implemented yet.

src/memory/consspaceobject.c

@@ -39,6 +39,12 @@ struct cons_pointer privileged_keyword_location = NIL;
  */
 struct cons_pointer privileged_keyword_payload = NIL;
 
+/**
+ * Keywords used when constructing exceptions: `:payload`. Instantiated in 
+ * `init.c`, q.v.
+ */
+struct cons_pointer privileged_keyword_cause = NIL;
+
 /**
  * True if the value of the tag on the cell at this `pointer` is this `value`,
  * or, if the tag of the cell is `VECP`, if the value of the tag of the