[value] fix-up handling of recursive values in the library.
[value] fix-up handling of recursive values in the library.

Before, we had stuff like Rec(n,Ext(n ... )) which was bad.
Now we always have Rec(n,...) (ie. no more extra Ext)

mirage/dyntype,avsm/ocaml-orm-sqlite,mirage/orm,avsm/ocaml-orm-sqlite,mirage/orm