Support Ed25519 keys and signatures:

Recognize a new JSON parameter `key_type` in handlers for wallet_propose
and sign/submit.  In addition to letting the caller to specify either of
secp256k1 or ed25519, its presence prohibits the (now-deprecated) use of
heuristically polymorphic parameters for secret data -- the `passphrase`
parameter to wallet_propose will be not be considered as an encoded seed
value (for which `seed` and `seed_hex` should be used), and the `secret`
parameter to sign and submit will be obsoleted entirely by the same trio
above.

* Use constants instead of literals for JSON parameter names.
* Move KeyType to its own unit and add string conversions.
* RippleAddress
  * Pass the entire message, rather than a hash, to accountPrivateSign()
    and accountPublicVerify().
  * Recognize a 33-byte value beginning with 0xED as an Ed25519 key when
    signing and verifying (for accounts only).
  * Add keyFromSeed() to generate an Ed25519 secret key from a seed.
  * Add getSeedFromRPC() to extract the seed from JSON parameters for an
    RPC call.
  * Add generateKeysFromSeed() to produce a key pair of either type from
    a seed.
* Extend Ledger tests to cover both key types.

src/ripple/protocol/impl/STTx.cpp

@@ -152,6 +152,14 @@ STTx::getMentionedAccounts () const
     return accounts;
 }
 
+static Blob getSigningData (STTx const& that)
+{
+    Serializer s;
+    s.add32 (HashPrefix::txSign);
+    that.add (s, false);
+    return s.getData();
+}
+
 uint256
 STTx::getSigningHash () const
 {
@@ -178,8 +186,7 @@ Blob STTx::getSignature () const
 
 void STTx::sign (RippleAddress const& private_key)
 {
-    Blob signature;
-    private_key.accountPrivateSign (getSigningHash (), signature);
+    Blob const signature = private_key.accountPrivateSign (getSigningData (*this));
     setFieldVL (sfTxnSignature, signature);
 }
 
@@ -196,7 +203,7 @@ bool STTx::checkSign () const
             RippleAddress n;
             n.setAccountPublic (getFieldVL (sfSigningPubKey));
 
-            sig_state_ = n.accountPublicVerify (getSigningHash (),
+            sig_state_ = n.accountPublicVerify (getSigningData (*this),
                 getFieldVL (sfTxnSignature), fullyCanonical);
         }
         catch (...)