mk: numbero / stringo / symbolo type predicates

Ground-only type tests via project. Each succeeds iff its argument
walks to the corresponding host value type. Composes with membero for
type-filtered enumeration:

  (fresh (x) (membero x (list 1 "a" 2 "b" 3)) (numbero x) (== q x))
    -> (1 2 3)

12 new tests, 328/328 cumulative. Caveat: SX keywords are strings, so
(stringo :k) succeeds.

lib/minikanren/intarith.sx

@@ -54,3 +54,9 @@
     (project
       (a b)
       (if (and (number? a) (and (number? b) (not (= a b)))) succeed fail))))
+
+(define numbero (fn (x) (project (x) (if (number? x) succeed fail))))
+
+(define stringo (fn (x) (project (x) (if (string? x) succeed fail))))
+
+(define symbolo (fn (x) (project (x) (if (symbol? x) succeed fail))))