Remove datatype context from 'DecimalRaw'.

@PaulJohnson, this patch removes the datatype context (the `Integral i` constraint) from the definition of `DecimalRaw i`. I also removed the constraint from the definition of the `NFData` instance.

There are several reasons to do this. First, the context doesn't do anything; the library behaves the same without it. Second, datatype contexts were removed from the Haskell 2011 standard and are now broadly considered a bad idea. Third, the context prevents doing some useful things with `DecimalRaw` and `Decimal`, e.g., using `GHC.Generics` to derive instances automatically.