add to-do for memory management

src/polluck_blockchain/placeholder_native.pyx

@@ -164,6 +164,8 @@ cdef class Blockchain:
     def __init__(self, *args, **kwargs):
         pass
     
+    # TODO add entries of map
+    # ownership is typically not transferred from the Blockchain extension class
     def __dealloc__(self):
         if self._chain is not NULL:
             del self._chain