xo-alloc2: + DArena.scrub() for diagnostics

include/xo/arena/DArena.hpp

@@ -229,6 +229,9 @@ namespace xo {
             void restore(Checkpoint ckp) noexcept { free_ = ckp.free_; }
 
 
+            /** zero out all allocated memory. Likely use case is diagnostics **/
+            void scrub() noexcept;
+
             /** discard all allocated memory, return to empty state
              *  Promise:
              *  - committed memory unchanged

src/arena/DArena.cpp

@@ -583,6 +583,12 @@ namespace xo {
             return true;
         } /*expand*/
 
+        void
+        DArena::scrub() noexcept
+        {
+            ::memset(this->lo_, 0, this->free_ - this->lo_);
+        }
+
         void
         DArena::clear() noexcept
         {