RFR 8171279: Support X25519 and X448 in TLS 1.3

[Adam Petcher]

Webrev: http://cr.openjdk.java.net/~apetcher/8171279/webrev.00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8171279

Please review the following change to add support for X25519 and X448 
(XDH) to TLS 1.3. This change includes some refactoring to remove code 
that was duplicated for DH and ECDH, and to avoid adding more for XDH. 
In addition to running the included regression test, I tested by 
connecting to an openssl server and confirmed that the connection was 
established using TLS 1.3 and X25519/X448.

Here are some detailed notes:

*) The NamedGroupFunctions class was added to hold the functions that 
are needed for key agreement with some named group. Most of the 
group-specific code was moved into subclasses of NamedGroupFunctions. 
This allowed me to remove a bunch of code like "if (type == ECDHE) {...} 
else if (type == FFDHE) {...}".
*) There are a couple of files in the webrev with no changes due to a 
webrev issue. Please ignore them.
*) I moved some code related to XDH parameters and encoding into 
java.base. ECUtil now has code to encode/decode XDH public keys, and the 
XECParameters file was moved into java.base/sun.security.util. This 
organization is similar to how CurveDB and NamedCurve are in java.base, 
and it allows the TLS implementation to encode/decode keys without using 
the jdk.crypto.ec module.