pool: add accessor for base address

pool.hpp

@@ -160,7 +160,15 @@ namespace util {
 
 
         // Indexing
-        size_t index (const T*) const;
+
+
+        /// returns the base address of the allocation.
+        ///
+        // guaranteed to point to the first _possible_ allocated value;
+        // however it may not be _live_ at any given moment. provided to
+        // facilitate indexing.
+        T* base (void) { return reinterpret_cast<T*> (m_head); }
+        T const* base (void) const { return reinterpret_cast<T const*> (m_head); }
 
         T& operator[] (size_t idx) &;