allow xeno for vdict length type

xenomorph/xenomorph/scribblings/xenomorph.scrbl

@@ -1306,7 +1306,7 @@ The versioned dict is a format derived from @racket[x:dict%] that contains multi
 (version-type?
 [x any/c])
 boolean?]{
-Whether @racket[x] can be used as the version type of a versioned dict. Valid types are @racket[integer?], @racket[procedure?], @racket[xenomorphic?], or @racket[symbol?].
+Whether @racket[x] can be used as the version type of a versioned dict. Valid types are @racket[integer?], @racket[procedure?], or @racket[xenomorphic?].
 }
 
 

xenomorph/xenomorph/versioned-dict.rkt

@@ -15,7 +15,7 @@ https://github.com/mbutterick/restructure/blob/master/src/VersionedStruct.coffee
 |#
 
 (define (version-type? x)
-  (and x (length-resolvable? x)))
+  (and x (or (length-resolvable? x) (xenomorphic? x))))
 
 (define x:versioned-dict%
   (class x:dict%