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/app/paths/PathRequest.cpp

@@ -171,7 +171,7 @@ bool PathRequest::isValid (RippleLineCache::ref crCache)
                 // no destination account
                 jvDestCur.append (Json::Value ("XRP"));
 
-                if (!saDstAmount.isNative ())
+                if (!saDstAmount.native ())
                 {
                     // only XRP can be send to a non-existent account
                     bValid = false;