Address issues identified by external review:

* RIPD-1617, RIPD-1619, RIPD-1621:
  Verify serialized public keys more strictly before
  using them.

* RIPD-1618:
    * Simplify the base58 decoder logic.
    * Reduce the complexity of the base58 encoder and
      eliminate a potential out-of-bounds memory access.
    * Improve type safety by using an `enum class` to
      enforce strict type checking for token types.

* RIPD-1616:
  Avoid calling `memcpy` with a null pointer even if the
  size is specified as zero, since it results in undefined
  behavior.

Acknowledgements:
Ripple thanks Guido Vranken for responsibly disclosing these
issues.

Bug Bounties and Responsible Disclosures:
We welcome reviews of the rippled code and urge researchers
to responsibly disclose any issues that they may find. For
more on Ripple's Bug Bounty program, please visit:
https://ripple.com/bug-bounty

src/ripple/protocol/impl/SecretKey.cpp

@@ -87,10 +87,8 @@ public:
         std::memcpy(ui.data(), seed.data(), seed.size());
         auto gsk = generatePrivateDeterministicKey(gen_, ui, ordinal);
         auto gpk = generatePublicDeterministicKey(gen_, ordinal);
-        SecretKey const sk(Slice
-        { gsk.data(), gsk.size() });
-        PublicKey const pk(Slice
-        { gpk.data(), gpk.size() });
+        SecretKey const sk(Slice{ gsk.data(), gsk.size() });
+        PublicKey const pk(Slice{ gpk.data(), gpk.size() });
         beast::secure_erase(ui.data(), ui.size());
         beast::secure_erase(gsk.data(), gsk.size());
         return {pk, sk};
@@ -243,7 +241,7 @@ derivePublicKey (KeyType type, SecretKey const& sk)
             LogicError("derivePublicKey: secp256k1_ec_pubkey_create failed");
 
         unsigned char pubkey[33];
-        size_t len = sizeof(pubkey);
+        std::size_t len = sizeof(pubkey);
         if(secp256k1_ec_pubkey_serialize(
                 secp256k1Context(),
                 pubkey,
@@ -252,8 +250,7 @@ derivePublicKey (KeyType type, SecretKey const& sk)
                 SECP256K1_EC_COMPRESSED) != 1)
             LogicError("derivePublicKey: secp256k1_ec_pubkey_serialize failed");
 
-        return PublicKey{Slice{pubkey,
-            static_cast<std::size_t>(len)}};
+        return PublicKey{Slice{ pubkey, len }};
     }
     case KeyType::ed25519:
     {