Add an arena out-of-memory handler

example/arena.c

@@ -9,7 +9,8 @@ typedef struct {
 
 int main(void)
 {
-    Arena arena = gebs_arena_get(4*1024);
+    /* Arena arena = gebs_arena_get(512); */
+    Arena arena = gebs_arena_get(128);
     defer { arena_destroy(&arena); }
 
     Ints ints = {0};