added "list?" builtin

proc.go

@@ -110,3 +110,12 @@ func length(vals []interface{}) (interface{}, error) {
 func list(vals []interface{}) (interface{}, error) {
 	return sexp(vals), nil
 }
+
+func islist(vals []interface{}) (interface{}, error) {
+	if err := checkArity(1, vals, "list?"); err != nil {
+		return nil, err
+	}
+
+	_, ok := vals[0].(sexp)
+	return ok, nil
+}

skeam.go

@@ -29,6 +29,7 @@ var universe = &environment{map[symbol]interface{}{
 	"/":      builtin(division),
 	"length": builtin(length),
 	"list":   builtin(list),
+	"list?":  builtin(islist),
 	"not":    builtin(not),
 	"begin":  special(begin),
 	"define": special(define),