mk: prefixo + suffixo — appendo-derived sublist relations

Two-line definitions over appendo:
  (prefixo p l) ≡ ∃rest. (appendo p rest l)
  (suffixo s l) ≡ ∃front. (appendo front s l)

Both enumerate all prefixes/suffixes when called with a fresh first
arg, and serve as decision relations when called with both grounded.

9 new tests, 398/398 cumulative.

lib/minikanren/relations.sx

@@ -68,6 +68,11 @@
 
 (define palindromeo (fn (l) (fresh (rev) (reverseo l rev) (== l rev))))
 
+(define prefixo (fn (p l) (fresh (rest) (appendo p rest l))))
+
+(define suffixo (fn (s l) (fresh (front) (appendo front s l))))
+
+
 (define
   lengtho
   (fn
@@ -84,7 +89,6 @@
       ((conso a l p))
       ((fresh (h t pt) (conso h t l) (conso h pt p) (inserto a t pt))))))
 
-
 (define
   permuteo
   (fn