lazy conses: support state in car and cdr.

Here we allow application code to take advantage of a trick
already used internally.

When a lazy cons cell is created, we can temporarily put
state information into its car and cdr fields.

When these fields are accessed normally, by the car and cdr
function, the lazy cons' update function will be invoked,
which will populate these fields.

If we have a way for that function to retrieve the existing
values of those fields, then the function can treat those
fields as state information: it can retrieve the values into
temporary local variables, overwrite the original values,
and then propagate the state information into the car and cdr
into the next lazy cons cell being added.

Thus lazy list generation that needs two cells of state or
less does not require the allocation of a context object.

* eval.c (eval_init): make-lazy-cons becomes a three-argument
function with two optional parameters.  New functions
lcons-car and lcons-cdr are registered.

* lib.c (make_lazy_cons_pub): New function, wrapping
make_lazy_cons_car_cdr with argument defaulting.
(lcons_car, lcons_cdr): New functions.

* lib.h (make_lazy_cons_pu, lcons_car, lcons_cdr): Declared.

* txr.1: Updated doc of make-lazy-cons regarding new
arguments. Documented lcons-car and lcons-cdr.

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