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/app/PayChan_test.cpp

@@ -299,7 +299,7 @@ struct PayChan_test : public beast::unit_test::suite
             // Try to claim more than authorized
             auto const preBob = env.balance (bob);
             STAmount const authAmt = chanBal + XRP (500);
-            STAmount const reqAmt = authAmt + 1;
+            STAmount const reqAmt = authAmt + STAmount{1};
             assert (reqAmt <= chanAmt);
             auto const sig =
                 signClaimAuth (alice.pk (), alice.sk (), chan, authAmt);