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/KeyGeneration_test.cpp

@@ -308,7 +308,7 @@ public:
             (keyType ? *keyType : "no key_type"));
 
         auto const publicKey = parseBase58<PublicKey>(
-            TokenType::TOKEN_ACCOUNT_PUBLIC, strings.public_key);
+            TokenType::AccountPublic, strings.public_key);
         BEAST_EXPECT(publicKey);
 
         if (!keyType)