mk: even-i / odd-i — host-arithmetic parity goals

Two-line definitions over project + host even?/odd?. Ground-only — no
relational behaviour, but they pair cleanly with membero for filtered
enumeration:
  (fresh (x) (membero x (list 1 2 3 4 5 6)) (even-i x) (== q x))
    -> (2 4 6)

5 new tests, 416/416 cumulative.

lib/minikanren/tests/intarith.sx

@@ -86,4 +86,18 @@
       (== q x)))
   (list 1 2))
 
+(mk-test "even-i-pos" (run* q (even-i 4)) (list (make-symbol "_.0")))
+
+(mk-test "even-i-neg" (run* q (even-i 5)) (list))
+
+(mk-test "odd-i-pos" (run* q (odd-i 7)) (list (make-symbol "_.0")))
+
+(mk-test "odd-i-neg" (run* q (odd-i 4)) (list))
+
+(mk-test
+  "even-i-filter"
+  (run* q (fresh (x) (membero x (list 1 2 3 4 5 6)) (even-i x) (== q x)))
+  (list 2 4 6))
+
 (mk-tests-run!)
+