Remove hard-coded encodings from X25519 keys.

[juergw]

Instead, let BoringSSL do the parsing and the serialization.

Also, we replace a custom parsing function for X25519 with two generic functions calls in NativeCrypto.
This allows us to remove this custom function.

[juergw]

This doesn't work, because for some backwards compatibility, we need to ignore extra bytes at the end.

So I'll close this.