ECC support

Creating this pull request because right now there's not even an issue for discussion.

So I'm not sure if this should be in OK 2.9 or not - it *is* rather off track from our original plans. On the other hoof, it's there and mostly done, just needs more testing so getting the maturity in for OK3.0 would be a nice thing. [Then again...](https://www.schneier.com/blog/archives/2013/09/the_nsa_is_brea.html#c1675929)