rct amount key modified as per luigi1111's recommendations

This allows the key to be not the same for two outputs sent to
the same address (eg, if you pay yourself, and also get change
back). Also remove the key amounts lists and return parameters
since we don't actually generate random ones, so we don't need
to save them as we can recalculate them when needed if we have
the correct keys.

src/crypto/crypto.cpp

@@ -151,7 +151,7 @@ namespace crypto {
     return true;
   }
 
-  static void derivation_to_scalar(const key_derivation &derivation, size_t output_index, ec_scalar &res) {
+  void crypto_ops::derivation_to_scalar(const key_derivation &derivation, size_t output_index, ec_scalar &res) {
     struct {
       key_derivation derivation;
       char output_index[(sizeof(size_t) * 8 + 6) / 7];