Relax restrictions on dwim places.

No longer require the leftmost expression in a dwim place to
itself be a place, except when the expression evaluates to
a list, and the list is subject to an element deletion or
a range operation.

* eval.c (eval_init): Register dwim-set and dwim-del with
one additional argument that the C functions now take.

* lib.c (dwim_set, dwim_del): Take a new place_p argument
which informs these functions whether the object they
are operating on came from a syntactic place. The forbidden
situations are diagnosed based on this flag: modification
of the subrange of a list, or deletion of a list ref.
Some error messages reworded.

* lib.h (dwim_set, dwim_del): Declarations updated.

* share/txr/stdlib/place.tl (defplace dwim): Produce a
different update, clobber and delete expansion when
the obj-place form isn't a place. In the non-place case,
do not assign the result of the sys:dwim-set or
sys:dwim-del operation back obj-place. Furthermore,
pass a Boolean flag to sys:dwim-set and sys:dwim-del
indicating which situation is the case: did the object
argument come from a place or non-place.

* txr.1: Documentation updated.

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