Ref10 scalar multiplication and JCA

This pull request is a near-complete rewrite of `Ed25519.java`.
- The code uses the JCA and can be included in security providers.
- Support has been added for other future EdDSA parameter specs.
- The single and double scalar multiplication methods from ref10 have been ported over.
- The tests have been rewritten in JUnit4.

Signing takes about 2.2ms, and verification takes about 3.3ms.