ecdh: Use library functions for Ed25519->X25519 conversion

[Yawning]

Since this was using agl's PublicKeyToCurve25519/PrivateKeyToCurve25519, the X25519 implementation being used provides equivalent functionality (
oasisprotocol/ed25519@267bbda), so it is possible to avoid having to do it with math/big.

Note: The differences referred to in the commit message that adds the functionality have to do with the function prototypes, the conversion routines should produce identical results.