Fix tpie::packed_array cannot compile with MSVC 19.28+

Microsoft's standard library assumes that a const version of
tpie::packed_array::iterator and similar only are immutable with respect to the
index, not the value in said index. That is, they assume the iterator reflects a
'T* const' semantics, whereas the prior version implemented a 'const T* const'
semantics."

tpie/packed_array.h

@@ -300,10 +300,18 @@ class packed_array
 		iter_return_type & operator*()
 		{ return elm; }
 
+		///////////////////////////////////////////////////////////////////////
+		iter_return_type & operator*() const
+		{ return const_cast<iter_return_type&>(elm); }
+
 		///////////////////////////////////////////////////////////////////////
 		iter_return_type * operator->()
 		{ return &elm; }
 
+		///////////////////////////////////////////////////////////////////////
+		iter_return_type * operator->() const
+		{ return &elm; }
+
 		///////////////////////////////////////////////////////////////////////
 		iter_base & operator=(const iter_base & o)
 		{