enable printing cyclic data structures; closes #11

As the last piece, tests in 072.

regtests/072-cycles.lisp

+;;; printing (or rather string conversion) of cyclic data structures
+
+;; Testing this with an environment is difficult, and I don't know where I
+;; (test-is "cycles env" ...)
+
+(test-is "cycles pair" (let ((a '(3 4 5 6)))
+                         (setf (cadr a) a)
+                         a)
+         "(3 [...] 5 6)")
+
+(defstruct self-holder "can hold its liquor and itself" this that)
+(test-is "cycles struct" (let ((s (make-self-holder :this 19 :that 23)))
+                           (setf (self-holder-this s) s)
+                           s)
+         "#S(self-holder this [...] that 23)")
+
+(test-is "cycles table" (let ((tbl #:()))
+                          (tbl tbl tbl)
+                          (tbl tbl))
+         "#:(([...] . [...]))")
+
+(test-is "cycles vector" (let ((v #(4 1 2))
+                               (w #(4 1 1)))
+                           (setf (aref v 1) w)
+                           (setf (aref w 2) v)
+                           w)
+         "#(4 1 #(4 [...] 2))")
+
+(defstruct foo "each must play a part" bar ble quu)
+(test-is "cycles indirect" (let* ((a #:((1 . 2) (3 . 4)))
+                                  (b (list 5 6 7 a))
+                                  (c (vector 8 9 10 b))
+                                  (f (make-foo :bar 11 :ble 12 :quu c)))
+                             (a 3 f)
+                             a)
+         "#:((1 . 2)(3 . #S(foo bar 11 ble 12 quu #(8 9 10 (5 6 7 [...])))))")
+
+(done-testing)
+