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/rpc/NoRippleCheck_test.cpp

@@ -112,7 +112,7 @@ class NoRippleCheck_test : public beast::unit_test::suite
           // parsing as a seed to fail
             Json::Value params;
             params[jss::account] =
-                toBase58 (TokenType::TOKEN_NODE_PRIVATE, alice.sk());
+                toBase58 (TokenType::NodePrivate, alice.sk());
             params[jss::role] = "user";
             params[jss::ledger] = "current";
             auto const result = env.rpc ("json", "noripple_check",