Use Number for IOUAmount and STAmount arithmetic

* Guarded by amendment fixUniversalNumber
* Produces slightly better accuracy in some computations.

src/ripple/app/tx/impl/Transactor.cpp

@@ -782,6 +782,7 @@ Transactor::operator()()
     JLOG(j_.trace()) << "apply: " << ctx_.tx.getTransactionID();
 
     STAmountSO stAmountSO{view().rules().enabled(fixSTAmountCanonicalize)};
+    NumberSO stNumberSO{view().rules().enabled(fixUniversalNumber)};
 
 #ifdef DEBUG
     {