get rid of use of Seq

[stew]

we are passing the Seq type around in a bunch of places. This type is an abomination we should avoid, since Seq doesn't tell us if the set is bounded, and doens't tell us if the set is immutable. There are, in fact, places in our code where we end up with a mutable Buffer. Even worse though is that there are places that we are calling .length on a List. We should probably just converge on Vector all these places.

[kevinmeredith]

Would using the immutable Seq type (per http://hseeberger.github.io/blog/2013/10/25/attention-seq-is-not-immutable/) be a reasonable replacement?