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/basics/FeeUnits_test.cpp

@@ -344,8 +344,7 @@ public:
     void run() override
     {
         BEAST_EXPECT(INITIAL_XRP.drops() == 100'000'000'000'000'000);
-        BEAST_EXPECT(INITIAL_XRP ==
-            XRPAmount{ 100'000'000'000'000'000 });
+        BEAST_EXPECT(INITIAL_XRP == XRPAmount{100'000'000'000'000'000});
 
         testTypes();
         testJson();