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/rpc/handlers/GatewayBalances.cpp

@@ -93,7 +93,7 @@ Json::Value doGatewayBalances (RPC::Context& context)
             if (j.isString())
             {
                 auto const pk = parseBase58<PublicKey>(
-                    TokenType::TOKEN_ACCOUNT_PUBLIC,
+                    TokenType::AccountPublic,
                     j.asString ());
                 if (pk)
                 {