Make loaded set order deterministic by using LinkedHashSet.

This can make it easier to write test cases, since set elements will
come in the same order as in the persisted set.

It is also consistent with Groovy, which considers LinkedHashSet the default Set type
(e.g. [] as Set returns a LinkedHashSet in recent Groovy versions).
Merge pull request #88 from tomwidmer/deterministic-sets

Make loaded set order deterministic by using LinkedHashSet.