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.h

@@ -68,6 +68,12 @@ extern struct cons_pointer privileged_keyword_location;
  */
 extern struct cons_pointer privileged_keyword_payload;
 
+/**
+ * Keywords used when constructing exceptions: `:cause`. Instantiated in 
+ * `init.c`.
+ */
+extern struct cons_pointer privileged_keyword_cause;
+
 /**
  * An unallocated cell on the free list - should never be encountered by a Lisp
  * function.
@@ -456,6 +462,8 @@ struct stack_frame {
     struct cons_pointer function;
     /** the number of arguments provided. */
     int args;
+    /** the depth of the stack below this frame */
+    int depth;
 };
 
 /**