Reduce STAmount public interface (RIPD-867):

* Implement subtraction as addition to the additive inverse
* Do not allow comparison with, addition to or subtraction from integers
* Remove unused functions
* Convert member functions to free functions
* Isolate unit-test specific code into the unit test

src/ripple/rpc/handlers/AccountLines.cpp

@@ -45,7 +45,7 @@ void addLine (Json::Value& jsonLines, RippleState const& line)
     // Amount reported is negative if other account holds current
     // account's IOUs.
     jPeer[jss::balance] = saBalance.getText ();
-    jPeer[jss::currency] = saBalance.getHumanCurrency ();
+    jPeer[jss::currency] = to_string (saBalance.issue ().currency);
     jPeer[jss::limit] = saLimit.getText ();
     jPeer[jss::limit_peer] = saLimitPeer.getText ();
     jPeer[jss::quality_in]

src/ripple/rpc/impl/TransactionSign.cpp

@@ -400,7 +400,7 @@ static Json::Value checkPayment(
             saSendMax.setIssuer (raSrcAddressID.getAccountID ());
         }
 
-        if (saSendMax.isNative () && amount.isNative ())
+        if (saSendMax.native () && amount.native ())
             return RPC::make_error (rpcINVALID_PARAMS,
                 "Cannot build XRP to XRP paths.");
 
@@ -486,7 +486,7 @@ checkTxJsonFields (
 
     // Check for current ledger.
     if (verify && !getConfig ().RUN_STANDALONE &&
-        (apiFacade.getValidatedLedgerAge() > 
+        (apiFacade.getValidatedLedgerAge() >
           Tuning::maxValidatedLedgerAge))
     {
         ret.first = rpcError (rpcNO_CURRENT);