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.
* Allow `passphrase` to be a seed encoded in any of three formats or a
literal passphrase.
* Recognize the absence of `passphrase` as requesting a random seed.
Extract walletPropose() and keypairForSignature() as separately factored
functions (from doWalletPropose() and transactionSign() respectively) to
facilitate unit testing.
