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/protocol/STAmount_test.cpp

@@ -78,7 +78,7 @@ public:
         STAmount num (noIssue(), n);
         STAmount den (noIssue(), d);
         STAmount mul (noIssue(), m);
-        STAmount quot = divide (n, d, noIssue());
+        STAmount quot = divide(STAmount(n), STAmount(d), noIssue());
         STAmount res = roundSelf (multiply (quot, mul, noIssue()));
 
         BEAST_EXPECT(! res.native ());