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/init.c

@@ -96,6 +96,9 @@ void maybe_bind_init_symbols(  ) {
     if ( nilp( privileged_keyword_payload ) ) {
         privileged_keyword_payload = c_string_to_lisp_keyword( L"payload" );
     }
+    if ( nilp( privileged_keyword_cause)) {
+        privileged_keyword_cause = c_string_to_lisp_keyword(L"cause");
+    }
 }
 
 void free_init_symbols(  ) {