add guarded objects that are sampled (and fit a size range). guarded sample rate etc can be set per heap as well as defaulted with options

include/mimalloc/types.h

@@ -507,7 +507,7 @@ struct mi_heap_s {
   size_t                guarded_size_max;                    // maximal size for guarded objects
   size_t                guarded_sample_rate;                 // sample rate (set to 0 to disable guarded pages)
   size_t                guarded_sample_seed;                 // starting sample count
-  size_t                guarded_sample_count;                // current sample count (wraps at `sample_rate`)
+  size_t                guarded_sample_count;                // current sample count (counting down to 0)
   #endif
   mi_page_t*            pages_free_direct[MI_PAGES_DIRECT];  // optimize: array where every entry points a page with possibly free blocks in the corresponding queue for that size.
   mi_page_queue_t       pages[MI_BIN_FULL + 1];              // queue of pages for each size class (or "bin")