2.10.x binary incompatibility (SeqFactory.unapplySeq)

This commit [1] introduced a binary incompatibility in the collections
library, as shown here [2]. I remember a related discussion between
with Paul/Alexander, but I can't dig it up.

This makes it a bit hard to validate the recent compiler performance
improvements on our larger projects, as we would need to update the
dependencies.

Let me know if I should just file a ticket. Hopefully a sprinkling of
@bridge will be enough.

-jason

[1] https://github.com/scala/scala/commit/e6c82caede9b72d859e5459912d4175ae5...
[2] https://github.com/retronym/scratch/tree/master/20110613