Another fix to print method circular printing.

Continuing on the theme of the previous patch, we now properly
detect the situation when the recursive call is re-introducing
duplicate references to objects that have already been sent to
the stream without at #<num>= label.  It's too late to print
those objects, so we throw an exception.

* lib.c (obj_print_impl): When we print an object that doesn't
have duplicates (its hash entry has a nil value), we replace
that value with the : symbol to indicate that the object has
already been printed.  (obj_hash_merge): New function,
factoring out the hash merging logic from obj_print,
introduced in parent commit. Here, we detect the case that the
recursive print call has submitted to us an object which was
already printed without a label: because it is associated with
the : symbol in the parent hash. This situation is a
show-stopper so we throw. We cannot attempt to print the
object in any manner because we can get into runaway
recursion.
(obj_print): Hash merging logic replaced with call to new
static function.

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