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/ripple/rpc/impl/TransactionSign.cpp

@@ -1154,7 +1154,7 @@ Json::Value transactionSubmitMultiSigned (
                 << " field.  Fees must be specified in XRP.";
             return RPC::make_error (rpcINVALID_PARAMS, err.str ());
         }
-        if (fee <= 0)
+        if (fee <= STAmount{0})
         {
             std::ostringstream err;
             err << "Invalid " << sfFee.fieldName