The original specification used a Collection so that a single SubmitRule
can return multiple SubmitRecords. The main use-case was inheritance and
being open to more future usages.
Looking at this today, this seems strange because a SubmitRecord keeps a
list of SubmitRequirements, so even if it is not allowed to return a
Collection, it can still contribute a number of SubmitRecords.
This does invalidate the original reasoning from using a Collection
which is why this commit replaces it with an Optional.
Change-Id: I0dce438504b30fe102f5b83af9f7976659f1fae8
7ab1b18ef8