## Backpack does not handle unlifted types

In the code snippet below, I attempt to use backpack with levity polymorphism:

```
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DataKinds #-}
unit number-unknown where
signature NumberUnknown where
import GHC.Types
data Number
plus :: Number -> Number -> Number
multiply :: Number -> Number -> Number
module NumberStuff where
import NumberUnknown
funcA :: Number -> Number -> Number
funcA x y = plus x (multiply x y)
unit number-int where
module NumberUnknown where
type Number = Int
plus :: Int -> Int -> Int
plus = (+)
multiply :: Int -> Int -> Int
multiply = (*)
unit number-unboxed-int where
module NumberUnknown where
import GHC.Prim
type Number = Int#
plus :: Int# -> Int# -> Int#
plus = (+#)
multiply :: Int# -> Int# -> Int#
multiply = (*#)
unit main where
dependency number-unknown[NumberUnknown=number-unboxed-int:NumberUnknown]
module Main where
import NumberStuff
main = putStrLn "Hello world!"
```

Compiling this with `ghc --backpack packer.bkp`

fails with the following error:

```
- Type constructor ‘Number’ has conflicting definitions in the module
and its hsig file
Main module: type Number = GHC.Prim.Int# :: TYPE 'GHC.Types.IntRep
Hsig file: data Number
The types have different kinds
- while checking that number-unboxed-int:NumberUnknown implements signature NumberUnknown in number-unknown[NumberUnknown=number-unboxed-int:NumberUnknown]
type Number = Int#
```

The error is pretty clear: `Number`

can only be instantiated by types of kind `Type`

(aka `TYPE LiftedRep`

). Even while remaining levity monomorphic, there doesn't seem to be a way to pick a different kind. For example, redefining `Number`

in the signature as

`data Number :: TYPE IntRep`

leads to the following immediate failure:

`Kind signature on data type declaration has non-* return kind TYPE 'IntRep`

I do not understand any of the internals of backpack, so I do not understand if there's anything fundamental that makes this impossible. Going one step further, I would like to be able to do something like this (the syntax here is not even currently valid for a backpack signature):

```
type MyRep :: RuntimeRep
data Number :: TYPE MyRep
```

This may be instantiated with something like this:

```
type MyRep = IntRep
type Number = Int#
```

And then end users would be able to monomorphize levity-polymorphic functions. This would be really neat because there is currently no way to do this in GHC.

So, I guess there are really two feature requests in here. One is the ability to use unlifted data types with backpack. The other is the ability to use backpack to monomorphize levity-polymorphic functions.

It seems unlikely to sneak this into GHC 8.2 but who knows!