Add a constant for the maximum number representable as a Ripple value

Add a constant for the maximum canonical value that can be expressed as a Ripple value.

It is possible to express larger values in both the binary and JSON formats, but I believe that these would be invalid since they exceed the cMaxOffset and cMaxValue constants. See: https://github.com/ripple/rippled/blob/develop/src/ripple/module/data/protocol/STAmount.h#L42-46