Work around https://github.com/golang/go/issues/13850
Work around https://github.com/golang/go/issues/13850

Serialize the file id to File struct map manually, rather than by passing it to gob/Encode() in its entirety, as the Encode() implementation recently changed to issue an error if the map is very large.

(I believe that this will fix skicka issue #98.)

google/skicka