ptr_container supports comparison of iterators of different types

[Rob Desbois <rob.desbois@…>]

The void_ptr_iterator, which handles much of the ptr_container iterator functionality, oddly supports comparison operations between iterators of differing types.

For example:

   ptr_vector<int>     vi;
   ptr_vector<string>  vs;
   bool eq = (vi.begin() == vs.begin());

The affected operators are: == != < <= > >=