theller: [_objects] will not be set if the instance is modified by a foreign function call, so this is not robust.

I thought about this before making my suggestion.  I believe people will be less surprized if objects returned from an ffi call print with funny numbers than getting c_char_p(NNNNN) after entering c_char_p("abc"). 

A few comments on the patch:

1. Since c_void_p is printed in decimal, shouldn't c_char_p be treated the same? If hexadecimal is prefered, maybe c_void_p should be changed.

2. Currently c_char_p(0) is printed as c_char_p(None), but the patch will change that to c_char_p(0).