Modify GC.stats to allow requesting specific stats

Author: mihails-strasuns-sociomantic

src/core/memory.d

@@ -680,8 +680,14 @@ struct GC
     /**
      * Returns runtime stats for currently active GC implementation
      * See `core.memory.GC.Stats` for list of available metrics.
+     *
+     * Params:
+     *    fields = optional bit flag argument which specifies which stats need
+     *      to be calculated. By default equals to "all fields". If some field
+     *      was not requested via bit flag, its value in returned `Stats` struct
+     *      will be undefined.
      */
-    static Stats stats() nothrow
+    static Stats stats(ulong fields = ulong.max) nothrow
     {
         return gc_stats(ulong.max);
     }