[PATCH] Implement print-circle, use it in the byte compiler.

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > GNU has had this for a while. Their implementation isn’t particularly
 > object-oriented, and I wrote the bulk of the below before we moved to GPLv3,
 > so it’s very different code. If anyone’s interested in writing tests, I can
 > commit this, but I won’t get to writing tests myself for a while, and I’m
 > not in a rush to commit the change without them.

Haven't we had a feature of numbering self-referential objects, and
printing references to those objects in subobjects as numerical
reference since the turn of the millennium or so?