This changes the reference counting convention for passing cyobjects
as arguments in a function call.

Before this commit, cyobjects used the same convention as pyobjects:
- The function borrows a reference on the argument from the caller
  and the caller keeps ownership of the object passed as argument,
  and must eventually decref it, even if it is a temporary rvalue
  that will not be reachable in the caller's scope after the call.
- If the function needs to take ownership of the argument, e.g. to
  store it, it must increment its reference count first, at which
  point the caller and callee both own a reference to the object.
- If the callee does not take ownership of the argument, it should
  not decrement its reference count at any point.

After this commit, the convention for cyobjects is as follows:
- The function steals the reference on the argument from the caller,
  and the caller should not decrement its reference count after the
  call.
- If the o...