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/test/app/Manifest_test.cpp

@@ -251,7 +251,7 @@ public:
                 std::string cfgManifest;
                 for (auto const& man : inManifests)
                     s1.push_back (toBase58(
-                        TokenType::TOKEN_NODE_PUBLIC, man->masterKey));
+                        TokenType::NodePublic, man->masterKey));
                 unl->load (emptyLocalKey, s1, keys);
 
                 m.save (dbCon, "ValidatorManifests",
@@ -428,7 +428,7 @@ public:
 
         {
             auto const valSecret = parseBase58<SecretKey>(
-                TokenType::TOKEN_NODE_PRIVATE,
+                TokenType::NodePrivate,
                 "paQmjZ37pKKPMrgadBLsuf9ab7Y7EUNzh27LQrZqoexpAs31nJi");
 
             // Format token string to test trim()