Changes needed for ECDSA

ASN.1 serialization was in my TODO list, but I realized that I can't directly serialize to `IntVal`s like the DSA or RSA types. The tricky part comes when trying to distinguish between `Point` and `PointO` and also between binary and prime curves.

It's going to take a while until I find some time to get familiar with ASN.1 (I don't find the specification too easy to understand), so you might not want to wait until I deal with ASN.1.