Process curve name when loading EC keys. Fixes #1336

Not super happy with `CURVE_TYPES` so if anyone has a better suggestion speak up.
I don't have a strong preference either way. When generating we know the curve immediately so that's why we do it this way right now. If we switch to lazy for consistency I'd say we should change the `_EllipticCurvePublicKey` and `_EllipticCurvePrivateKey` constructors to not take curve.