Make XRPAmount constructor explicit:

Remove the implicit conversion from int64 to XRPAmount. The motivation for this
was noticing that many calls to `to_string` with an integer parameter type were
calling the wrong `to_string` function. Since the calls were not prefixed with
`std::`, and there is no ADL to call `std::to_string`, this was converting the
int to an `XRPAmount` and calling `to_string(XRPAmount)`.

Since `to_string(XRPAmount)` did the same thing as `to_string(int)` this error
went undetected.

src/test/jtx/impl/amount.cpp

@@ -77,7 +77,7 @@ operator<< (std::ostream& os,
     if (amount.value().native())
     {
         // measure in hundredths
-        XRPAmount const c{dropsPerXRP.drops() / 100};
+        auto const c = dropsPerXRP.drops() / 100;
         auto const n = amount.value().mantissa();
         if(n < c)
         {