CTM-330: ordering of set values in expression evaluation

[davidangb]

Ticket: CTM-330

When evaluating an expression against a set of entities, the result does not respect the order of entities in the set.

Imagine you have some participants:

participant_id	name
1	Alice
2	Bob
3	Carol
4	Dan
5	Eve

And a participant set:

participant_set_id	participants
my-set	[1, 2, 3, 4, 4, 4, 4]

And you run a workflow against my-set using an expression of this.participants.name into a WDL input of Array[String].

Previously, you would have received an input that did not respect your set order:
[Bob, Dan, Carol, Alice]

Now, you will have received an input that exactly matches your set:
[Alice, Bob, Carol, Dan]

Note that in both cases, multiple references to the same entity are de-duplicated; "Dan" only appears once. This preserves behavior of pre-Quicksilver expression evaluation. I verified pre-Quicksilver behavior by checking out a commit before we deleted the legacy expression evaluation and adapting this PR's unit tests into that old commit.

Also note that the legacy pre-Quicksilver submissions also did not handle ordering correctly. So, this fix is technically a behavior change, though it feels correct.