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.
Avoid custom overflow code; simply use 128-bit math to
maintain precision and return a saturated 64-bit value
as the final result.
Disallow use of negative values in the `fee_mult_max`
and `fee_div_max` fields. This change could potentially
cause submissions with negative values that would have
previously succeeded to now fail.
