call-finalizers: allow recursion.

Code to invoke finalizers and remove them from the
list is consolidated: both the gc and call-finalizers
now use the same lower-level function.

Finalizers may now themselves call-finalizers; it is no longer
"unspecified behavior". This greatly improves the the TXR Lisp
support for RAII, since :fini handlers of objects can call
finalization on related objects. For instance a container
being finalized can call the finalizers of contained objects.

* gc.c (call_finalizers_impl): New function. Gathers all
eligible finalizer registrations into a local list,
first, removing them from the global list. Then does the
calls in a second pass. Also, relative to the original
functions which it replaces, this fixes an incorrect case:
when the list is of length 1 and contains an eligible
entry, in which case the global final_tail pointer is left
aiming at a local variable!
(is_reachable_final): New static function.
(call_finalizers): Use call_finalizers_impl, specifying
is_reachable_final as the predicate.
(is_matching_final): New static function.
(gc_call_finalizers): Use call_finalizers_impl, specifying
is_matching_final as the predicate.

* txr.1: Update documentation about call-finalizers.

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