Refactor AccountID (RIPD-953):

All AccountID functionality is removed from RippleAddress and
replaced with free functions. The AccountID to string conversion
cache is factored out as an explicit type with an instance in
the Application object. New base58 conversion functions are used,
with no dependence on OpenSSL.

All types and free functions related to AccountID are consolidated
into one header file. Routines to operate on "tokens" are also
introduced and consolidated into a single header file.

A token one of the cryptographic primitives used in Ripple:

    Secret Seed
    Server Public Key
    Server Secret Key
    Account ID
    Account Public Key
    Account Private Key

    and these deprecated primitives:

    Account Family Seed
    Account Family Generator

src/ripple/protocol/impl/STTx.cpp

@@ -25,6 +25,7 @@
 #include <ripple/protocol/STAccount.h>
 #include <ripple/protocol/STArray.h>
 #include <ripple/protocol/TxFlags.h>
+#include <ripple/protocol/types.h>
 #include <ripple/basics/Log.h>
 #include <ripple/basics/StringUtilities.h>
 #include <ripple/json/to_string.h>
@@ -120,36 +121,29 @@ STTx::getFullText () const
     return ret;
 }
 
-std::vector<RippleAddress>
+boost::container::flat_set<AccountID>
 STTx::getMentionedAccounts () const
 {
-    std::vector<RippleAddress> accounts;
+    boost::container::flat_set<AccountID> list;
 
     for (auto const& it : *this)
     {
         if (auto sa = dynamic_cast<STAccount const*> (&it))
         {
-            auto const na = sa->getValueNCA ();
-
-            if (std::find (accounts.cbegin (), accounts.cend (), na) == accounts.cend ())
-                accounts.push_back (na);
+            AccountID id;
+            assert(sa->isValueH160());
+            if (sa->getValueH160(id))
+                list.insert(id);
         }
         else if (auto sa = dynamic_cast<STAmount const*> (&it))
         {
             auto const& issuer = sa->getIssuer ();
-
-            if (isXRP (issuer))
-                continue;
-
-            RippleAddress na;
-            na.setAccountID (issuer);
-
-            if (std::find (accounts.cbegin (), accounts.cend (), na) == accounts.cend ())
-                accounts.push_back (na);
+            if (! isXRP (issuer))
+                list.insert(issuer);
         }
     }
 
-    return accounts;
+    return list;
 }
 
 static Blob getSigningData (STTx const& that)
@@ -226,11 +220,6 @@ void STTx::setSigningPubKey (RippleAddress const& naSignPubKey)
     setFieldVL (sfSigningPubKey, naSignPubKey.getAccountPublic ());
 }
 
-void STTx::setSourceAccount (RippleAddress const& naSource)
-{
-    setFieldAccount (sfAccount, naSource);
-}
-
 Json::Value STTx::getJson (int) const
 {
     Json::Value ret = STObject::getJson (0);
@@ -269,6 +258,7 @@ std::string STTx::getMetaSQL (std::uint32_t inLedger,
     return getMetaSQL (s, inLedger, TXN_SQL_VALIDATED, escapedMetaData);
 }
 
+// VFALCO This could be a free function elsewhere
 std::string
 STTx::getMetaSQL (Serializer rawTxn,
     std::uint32_t inLedger, char status, std::string const& escapedMetaData) const
@@ -281,7 +271,7 @@ STTx::getMetaSQL (Serializer rawTxn,
 
     return str (boost::format (bfTrans)
                 % to_string (getTransactionID ()) % format->getName ()
-                % getSourceAccount ().humanAccountID ()
+                % toBase58(getAccountID(sfAccount))
                 % getSequence () % inLedger % status % rTxn % escapedMetaData);
 }
 
@@ -338,7 +328,7 @@ STTx::checkMultiSign () const
     Serializer const dataStart (startMultiSigningData ());
 
     // We also use the sfAccount field inside the loop.  Get it once.
-    RippleAddress const txnAccountID = getFieldAccount (sfAccount);
+    auto const txnAccountID = getAccountID (sfAccount);
 
     // Determine whether signatures must be full canonical.
     ECDSA const fullyCanonical = (getFlags() & tfFullyCanonicalSig)
@@ -398,17 +388,16 @@ STTx::checkMultiSign () const
     */
 
     // We use this std::set to detect this form of double-signing.
-    std::set<RippleAddress> firstLevelSigners;
+    std::set<AccountID> firstLevelSigners;
 
     // SigningFors must be in sorted order by AccountID.
-    RippleAddress lastSigningForID;
-    lastSigningForID.setAccountID ("");
+    AccountID lastSigningForID = zero;
 
     // Every signature must verify or we reject the transaction.
     for (auto const& signingFor : multiSigners)
     {
-        RippleAddress const signingForID =
-            signingFor.getFieldAccount (sfAccount);
+        auto const signingForID =
+            signingFor.getAccountID (sfAccount);
 
         // SigningFors must be in order by account ID.  No duplicates allowed.
         if (lastSigningForID >= signingForID)
@@ -440,13 +429,12 @@ STTx::checkMultiSign () const
         }
 
         // SingingAccounts must be in sorted order by AccountID.
-        RippleAddress lastSigningAcctID;
-        lastSigningAcctID.setAccountID ("");
+        AccountID lastSigningAcctID = zero;
 
         for (auto const& signingAcct : signingAccounts)
         {
-            RippleAddress const signingAcctID =
-                signingAcct.getFieldAccount (sfAccount);
+            auto const signingAcctID =
+                signingAcct.getAccountID (sfAccount);
 
             // None of the multi-signers may sign for themselves.
             if (signingForID == signingAcctID)