add X25519 and X448
add X25519 and X448

added OIDs from draft-ietf-curdle-pkix-04
chose the strings "x25519" and "x448" for the SPKI as well as "ed25519" and "ed448" for the ECDSA flavors (this is based on the text in the draft, 'For the id-EdDSA25519 value use the string "Ed25519".  For id-EdDSA448 use "Ed448".')
google/der-ascii