mk: subo — contiguous sublist relation

Composes two appendos: l = front ++ s ++ back, equivalently
  (appendo front-and-s back l) and (appendo front s front-and-s).

Goal order matters: doing the (appendo ground:l) split first makes the
search finitary; the second appendo is then deterministic given
front-and-s and ground s. Reversing the order causes divergence on
failing inputs (the front search becomes unbounded).

7 new tests, 405/405 cumulative.

lib/minikanren/relations.sx

@@ -73,6 +73,15 @@
 (define suffixo (fn (s l) (fresh (front) (appendo front s l))))
 
 
+(define
+  subo
+  (fn
+    (s l)
+    (fresh
+      (front-and-s back front)
+      (appendo front-and-s back l)
+      (appendo front s front-and-s))))
+
 (define
   lengtho
   (fn