Added wrappers for the functions to convert ed25519 keys to curve25519

I saw that there were no wrappers for the relatively new crypto_sign_ed25519_pk_to_curve25519 and crypto_sign_ed25519_sk_to_curve25519 functions in libsodium so I created them.  While they have to do with both crypto_sign and crypto_box, they have the crypto_sign prefix so I put them into PublicKeyAuth.  The only modification I did to PublicKeyBox was to make the byte length constants public so that I didn't have to redefine them in the existing scope of PublicKeyAuth.