Methods for turning objects into sequences.

Struct objects can now define methods car, cdr and nullify.
With these, they can participate in operations on sequences.

* eval.h (car_s, cdr_s): Declared.

* lib.c (nullify_s): New symbol variable.
(car, cdr): Implement for struct objects via, respectively,
their car and cdr methods.
(tolist): Handle objects by mapping through identity.
(nullify): Implement for objects optionally: if an object
is a struct with a nullify method, use it, otherwise go
through default case of just returning the object.
(empty): Implement for objects that have nullify method.
(obj_init): Initialize nullify_s.

* struct.c (maybe_slot): New function.

* struct.h (maybe_slot): Declared.

* txr.1: Documented car, cdr and nullify method
convention.

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