feat(Data): add payload slice accessor payload

[elrafoon]

My use case is to parse rather large register block from modbus powermeter.

I do it by designing exactly matching struct, deriving zerocopy::FromBytes for it, and the just creating it from register payload bytes as received from the powermeter. Since zerocopy crate has support for endianness, it looks to me like nearly perfect way to parse structured data represented in flat modbus registers.

I see no reason to hide register payload from library user - let him exploit them :-)

Thanks.