EdDSA: Backport versions 0.2/0.3 from github:

- Change key encoding to match curdle draft
- Support key decoding based on curdle draft
- Implement true constant-time cmov()
- Add handling of X509Key-wrapped EdDSA keys (GitHub PR #47)
- Clarify that KeyPairGenerator takes a key size, not strength
- Javadocs
GitHub PR #58:
- Make GroupElement immutable by moving the pre-computed logic to the constructors,
  allowing the synchronized checking of whether the pre-computed logic had executed or not
  to be removed since it always has when it is used because those code paths
  are modified to request it at construction time.
- This allows getNegativeA() to be lazy, and doesn't need volatile due to the immutability
  (and final fields - this is important part of the contract with the JVM memory model).
- Remove synchronized contention from the named curve table get method.
- Generally remove use of the named curve table get method with a constant curve name
  in hot code paths in favour of using a new static constant for the curve spec.
Overall performance changes:
- Keygen 46% faster
- Signing 39% slower (due to cmov)
- Verify 2% faster

core/java/test/junit/net/i2p/crypto/eddsa/math/GroupElementTest.java

@@ -760,7 +760,7 @@ public class GroupElementTest {
         GroupElement A = new GroupElement(curve, Utils.hexToBytes("d4cf8595571830644bd14af416954d09ab7159751ad9e0f7a6cbd92379e71a66"));
         GroupElement B = ed25519.getB();
         GroupElement geZero = curve.getZero(GroupElement.Representation.P3);
-        geZero.precompute(false);
+        //geZero.precompute(false);
 
         // 0 * GE(0) + 0 * GE(0) = GE(0)
         assertThat(geZero.doubleScalarMultiplyVariableTime(geZero, zero, zero),
@@ -802,7 +802,7 @@ public class GroupElementTest {
             // Arrange:
             final GroupElement basePoint = ed25519.getB();
             final GroupElement g = MathUtils.getRandomGroupElement();
-            g.precompute(false);
+            //g.precompute(false);
             final FieldElement f1 = MathUtils.getRandomFieldElement();
             final FieldElement f2 = MathUtils.getRandomFieldElement();