parallel/stack: add clear and size queries

parallel/stack.hpp

@@ -85,6 +85,20 @@ namespace cruft::parallel {
             return true;
         }
 
+        std::size_t capacity (void) const { return m_store.size (); }
+        std::size_t size     (void) const { return m_cursor; }
+        bool empty (void) const { return m_cursor == 0; }
+
+
+        void clear (void)
+        {
+            std::lock_guard lk (m_lock);
+            for (auto &raw: store (contract::I_HAVE_LOCKED_THIS_STRUCTURE))
+                raw.~ValueT ();
+            m_cursor = 0;
+        }
+
+
         // DO NOT make this enum easier to use. It's supposed to be annoying
         // so that people don't use it.
         enum class contract { I_HAVE_LOCKED_THIS_STRUCTURE };