optional: provide support for x448
Currently we provide support for the relatively new elliptic curve x25519. This provides a security level of around 128bits. We should also provide the option for higher security levels such as 224, provided by x448. This currently depends on nettle supporting this curve's operations.
Related drafts/RFCs: https://tools.ietf.org/html/rfc7748 https://tools.ietf.org/html/draft-ietf-tls-rfc4492bis-07