Get rid of Seq

[loewenheim]

So it turns out that I made a mistake when I based sequents on Seq in the interest of being general. I have since learned the rule of thumb that one should use Vector in general, unless a cons list is more appropriate, then it's List. For this reason, I would like to get rid of all Seqs in data structures. 5cb5fba accomplishes this for sequents.

[gebner]

I have done some performance tests with Lists vs. Vectors in trepplein and was pleasantly surprised by the performance of Vector. I think we should go ahead with this change, but only for data structures. Functions should IMHO have the most general types (i.e. usually Traversable), this should avoid some unnecessary conversions for varargs.

[loewenheim]

Alright, I'll see about changing data structures then.