made digest id generation more robust

When allowing sharing, marshalling will produce different values for
structurally equal values depending on the way they were built. This
leads to unnecessary recomputations. This problem showed in an
unexpected setting: I was observing unnecessary recomputations when
switching from byte code compilation to native. I guess that due to
different code optimizations, value sharing was different in the two
cases.

lib/internals/workflow.ml

@@ -123,7 +123,7 @@ and token =
 and any = Any : _ t -> any
 
 let digest x =
-  Digest.to_hex (Digest.string (Marshal.to_string x []))
+  Digest.to_hex (Digest.string (Marshal.(to_string x [No_sharing])))
 
 let id : type s. s t -> string = function
   | Input { id ; _ } -> id