EdDSA cleanup (thx zzz)
 - Remove duplicate load3 and load4 methods
 - Change load3 return type to int
 - Comment out dead stores
 - Re-roll the add, subtract, and negate loops;
   there's no speed benefit or timing reason to unroll them
 - Check for field already set
 - Remove shifts by 0
Findbugs
Shorten Ed25519FieldElement.multiply() to fix Dalvik runtime VerifyError
Test to show that Utils.equal(byte[], byte[]) is broken