Implement sequence comprehensions

[robin-aws]

As proposed in dafny-lang/rfcs#10

[kjx]

yes please!

and either some way of enumerating (multi)sets, or at least an "ordered set" in a library?

this is not sorted - because ordering is on insertion order/position (like a seq) not element comparisons.
or you can think of it as a seq where equality is defined over the set of elements.

I've pretty much built one, but it's not at all pretty --- it's parallel primitive set, seq, and map, and assumes
elements have equality (which they MUST to go in a set in the first place, right?). I guess one could just
use set comprehensions on the seqs but keeping them parallel seems to work a little better.