OpenPGP v6 elliptic curve key formats

[vanitasvitae]

This PR adds support for parsing new OpenPGP v6 key algorithms X25519, X448, Ed25519, Ed448.

Note, that while some of these algorithms were already sort of supported in OpenPGP v4 (EDDSA_LEGACY), v6 properly introduces them with a changed encoding wire format: https://openpgp-wg.gitlab.io/rfc4880bis/#name-public-key-algorithms

To prevent duplicated code, I also introduced OctetArrayPublicBCPGKey and OctetArraySecretBCPGKey classes, which handle the byte array length enforcing etc.

[vanitasvitae]

I split this PR out of #1353 for better readability.

[dghgit]

Thanks for the patch! Now merged.

One minor revision, in the interests of economy I ended up unifying the OctetArray base classes. If there's a need to separate secret and public at this level we can always introduce it later, but for now it doesn't seem needed/