mk: lasto + init-o — last and not-last list relations

lasto: x is the final element of l. Direct base case (l = (x)) plus
recurse-on-cdr.

init-o: init is l without its last element. Base case for singleton:
(== init ()). Otherwise recurse, threading the head through to the
init result via conso.

Together with appendo, the round-trip init append (list last) = l
holds, which is exercised by an end-to-end test.

8 new tests, 344/344 cumulative.

lib/minikanren/relations.sx

@@ -156,3 +156,19 @@
     (conde
       ((fresh (a d) (conso a d l) (rel a)))
       ((fresh (a d) (conso a d l) (someo rel d))))))
+
+(define
+  lasto
+  (fn
+    (l x)
+    (conde
+      ((conso x (list) l))
+      ((fresh (a d) (conso a d l) (lasto d x))))))
+
+(define
+  init-o
+  (fn
+    (l init)
+    (conde
+      ((fresh (x) (conso x (list) l) (== init (list))))
+      ((fresh (a d d-init) (conso a d l) (conso a d-init init) (init-o d d-init))))))