mk: selecto — choose element + rest of list

Classic miniKanren relation. (selecto x rest l) holds when l contains
x at any position with `rest` being everything else. Direct base case
(l = (x . rest)) plus the skip-head recursion that threads the head
through to the result rest.

Run modes: enumerate every (x, rest) split; recover rest given an
element; recover an element given the rest; (and ground/all combinations).

6 new tests, 411/411 cumulative.

lib/minikanren/relations.sx

@@ -82,6 +82,14 @@
       (appendo front-and-s back l)
       (appendo front s front-and-s))))
 
+(define
+  selecto
+  (fn
+    (x rest l)
+    (conde
+      ((conso x rest l))
+      ((fresh (a d r) (conso a d l) (conso a r rest) (selecto x r d))))))
+
 (define
   lengtho
   (fn