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/rpc/Transaction_test.cpp

@@ -35,6 +35,7 @@ class Transaction_test : public beast::unit_test::suite
         testcase("Test Range Request");
 
         using namespace test::jtx;
+        using std::to_string;
 
         const char* COMMAND   = jss::tx.c_str();
         const char* BINARY    = jss::binary.c_str();