Reduce STAmount public interface (RIPD-867):

The STAmount class includes a number of functions which serve as thin
wrappers, which are unused or used only in one place, or which break
encapsulation by exposing internal implemenation details. Removing
such functions simplifies the interface of the class and ensures
consistency.

* getSNValue and getNValue are now free functions
* canonicalizeRound is no longer exposed
* Removed addRound and subRound
* Removed overloads of multiply, mulRound, divide and divRound

src/ripple/app/paths/Pathfinder.cpp

@@ -430,7 +430,7 @@ namespace {
 // total number of paths we have to evaluate.
 STAmount smallestUsefulAmount (STAmount const& amount, int maxPaths)
 {
-    return divide (amount, STAmount (maxPaths + 2), amount);
+    return divide (amount, STAmount (maxPaths + 2), amount.issue ());
 }
 
 } // namespace