Lists of capabilities not implemented yet

[talex5]

If you define a schema with List(Interface) then:

1. The types are wrong: it gives e.g. val a_get_list : t -> [`Interface_xxx] list instead of val a_get_list : t -> [`Interface_xxx] Capability.t list
2. The implementation generated is failwith "not implemented (type iface)".

As a workaround, you can use a list of structs containing interfaces instead.