Introduce py-ref.h

Introduce py-ref.h

This patch introduces class gdbpy_ref, which is a sort of smart
pointer that owns a single Python reference to a PyObject.  This class
acts a bit like unique_ptr, but also a bit like shared_ptr (in that
copies do what you might expect); I considered going solely with
unique_ptr but it seemed quite strange to have a unique_ptr that
actually manages a shared resource.

Subsequent patches use this new class to simplify logic in the Python
layer.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-ref.h: New file.