Add Storable Instances

[waivio]

I have lots of love for data-dword, but it would be great if you could add Storable instances.

I don't know Template Haskell, but the instances I made are quite repetitive.

My library has these orphan instances:

-- Imports for Storable Instance of Data.DoubleWord
import Foreign.Storable (Storable, sizeOf, alignment, peek, poke)  -- Used for Storable Instances of Data.DoubleWord
import Foreign.Ptr (Ptr, plusPtr, castPtr)  -- Used for dealing with Pointers for the Data.DoubleWord Storable Instance

-- Machine Integers and Operations
import Data.Int (Int64)  -- Import standard Int sizes
import Data.DoubleWord (Word128,Int128,Int256,fromHiAndLo,hiWord,loWord) -- Import large Int sizes
import Data.Word (Word64)

-- =====================================================================
-- ===                  Storable Instances                           ===
-- =====================================================================
--
-- Orphan Instance for Word128 using the DoubleWord type class
instance Storable (Word128) where
  sizeOf _ = 16
  alignment _ = 16
  peek ptr = do
    hi <- peek $ offsetInt 0
    lo <- peek $ offsetWord 1
    return $ fromHiAndLo hi lo
      where
        offsetInt i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)
        offsetWord i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)
  poke ptr int = do
    poke (offsetInt 0) (hiWord int)
    poke (offsetWord 1) (loWord int)
      where
        offsetInt i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)
        offsetWord i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)

-- Orphan Instance for Int128 using the DoubleWord type class
instance Storable (Int128) where
  sizeOf _ = 16
  alignment _ = 16
  peek ptr = do
    hi <- peek $ offsetInt 0
    lo <- peek $ offsetWord 1
    return $ fromHiAndLo hi lo
      where
        offsetInt i = (castPtr ptr :: Ptr Int64) `plusPtr` (i*8)
        offsetWord i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)
  poke ptr int = do
    poke (offsetInt 0) (hiWord int)
    poke (offsetWord 1) (loWord int)
      where
        offsetInt i = (castPtr ptr :: Ptr Int64) `plusPtr` (i*8)
        offsetWord i = (castPtr ptr :: Ptr Word64) `plusPtr` (i*8)

-- Orphan Instance for Int256 using the DoubleWord type class
instance Storable (Int256) where
  sizeOf _ = 32
  alignment _ = 32
  peek ptr = do
    hi <- peek $ offsetInt 0
    lo <- peek $ offsetWord 1
    return $ fromHiAndLo hi lo
      where
        offsetInt i = (castPtr ptr :: Ptr Int128) `plusPtr` (i*16)
        offsetWord i = (castPtr ptr :: Ptr Word128) `plusPtr` (i*16)
  poke ptr int = do
    poke (offsetInt 0) (hiWord int)
    poke (offsetWord 1) (loWord int)
      where
        offsetInt i = (castPtr ptr :: Ptr Int128) `plusPtr` (i*16)
        offsetWord i = (castPtr ptr :: Ptr Word128) `plusPtr` (i*16)
--