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/JsonFields.h

@@ -177,6 +177,7 @@ JSS ( issuer );                     // in: RipplePathFind, Subscribe,
                                     //     Unsubscribe, BookOffers
                                     // out: paths/Node, STPathSet, STAmount
 JSS ( key );                        // out: WalletSeed
+JSS ( key_type );                   // in/out: WalletPropose, TransactionSign
 JSS ( last );                       // out: RPCVersion
 JSS ( last_close );                 // out: NetworkOPs
 JSS ( ledger );                     // in: NetworkOPs, LedgerCleaner,
@@ -296,6 +297,7 @@ JSS ( search_depth );               // in: RipplePathFind
 JSS ( secret );                     // in: TransactionSign, WalletSeed,
                                     //     ValidationCreate, ValidationSeed
 JSS ( seed );                       // in: WalletAccounts, out: WalletSeed
+JSS ( seed_hex );                   // in: WalletPropose, TransactionSign
 JSS ( send_currencies );            // out: AccountCurrencies
 JSS ( seq );                        // in: LedgerEntry;
                                     // out: NetworkOPs, RPCSub, AccountOffers