xo-alloc: try to make commit happen at start of GC cycle

src/alloc/ArenaAlloc.cpp

@@ -144,6 +144,12 @@ namespace xo {
             }
         }
 
+        void
+        ArenaAlloc::reset(std::size_t need_z) {
+            this->clear();
+            this->expand(need_z);
+        }
+
         void
         ArenaAlloc::capture_object_statistics(capture_phase phase,
                                               ObjectStatistics * p_dest) const