API review for X25519/X448

[Adam Petcher]

Now that the JEP[1] for X25519/X448 key agreement is a candidate, we can 
proceed with the API and specification review. Please review the 
proposed API spec[2] and provide comments by the end of Saturday,  
January 13, anywhere on earth. At that point, I will combine your 
feedback with the initial feedback from the CSR group[3] and submit the 
API for final review by the CSR.

The only significant change to the API since our last discussion[4] is 
that I changed the names of the key specs and interfaces from "XDH..." 
to "XEC..." This makes them more general and reusable in things like 
XEdDSA[5] and other non-Diffie-Hellman cryptosystems based on the 
representations/operations defined in RFC 7748[6].

[1] http://openjdk.java.net/jeps/324
[2] https://bugs.openjdk.java.net/browse/JDK-8189806
[3] https://wiki.openjdk.java.net/display/csr/Main
[4] 
http://mail.openjdk.java.net/pipermail/security-dev/2017-September/016325.html
[5] https://signal.org/docs/specifications/xeddsa/
[6] https://tools.ietf.org/html/rfc7748