Add support for Ed25519 seeds encoded using ripple-lib:

When Ed25519 support was added to ripple-lib, a way to specify
whether a seed should be used to derive a "classic" secp256k1
keypair or a "new" Ed25519 keypair was needed, and the
requirements were that:

1. previously seeds would, correctly, generate a secp256k1
   keypair.
2. users would not have to know about whether the seed was
   used to generate a secp256k1 or an Ed25519 keypair.

To address these requirements, the decision was made to encode
the type of key within the seed and a custom encoding was
designed.

The encoding uses a token type of 1 and prefixes the actual
seed with a 2 byte header, selected to ensure that all such
keypairs will, when encoded, begin with the string "sEd".

This custom encoding is non-standard and was not previously
documented; as a result, it is not widely supported and other
sofware may treat such keys as invalid. This can make it
difficult for users that have stored such a seed to use
wallets or other tooling that is not based on ripple-lib.

This commit adds support to rippled for automatically
detecting and properly handling such seeds.

src/ripple/unity/crypto.cpp

@@ -20,7 +20,6 @@
 
 #include <ripple/crypto/impl/ec_key.cpp>
 #include <ripple/crypto/impl/GenerateDeterministicKey.cpp>
-#include <ripple/crypto/impl/KeyType.cpp>
 #include <ripple/crypto/impl/openssl.cpp>
 #include <ripple/crypto/impl/csprng.cpp>
 #include <ripple/crypto/impl/RFC1751.cpp>