Step 1: Convert Number to use 128-bit numbers internally

- Update the conversion points between Number and *Amount & STNumber.
- Tests probably don't pass.

include/xrpl/protocol/Issue.h

@@ -37,6 +37,9 @@ public:
     bool
     native() const;
 
+    bool
+    integral() const;
+
     friend constexpr std::weak_ordering
     operator<=>(Issue const& lhs, Issue const& rhs);
 };