## Improve `mkInteger` interface

`mkInteger`

is the current operation provided by the `Integer`

libraries to construct (large) integer values. The current type-signature is

```
mkInteger :: Bool -- sign-bit
-> [Int] -- absolute value in 31 bit chunks, least significant first
-> Integer
```

A somewhat pathological example of why this representation is not so nice is the following simple CAF

```
c :: Integer
c = 0xf000000000000000
```

that (this is for a 64bit arch!) gets compiled into the following STG:

```
==================== STG syntax: ====================
sat_sJQ :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [3];
sat_sJR :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJQ GHC.Types.[]];
sat_sJP :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [1610612736];
sat_sJS :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJP sat_sJR];
sat_sJO :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [0];
sat_sJT :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJO sat_sJS];
Foo.c :: GHC.Integer.Type.Integer
[GblId, Str=DmdType] =
\u srt:SRT:[0Y :-> GHC.Integer.Type.mkInteger, sJT :-> sat_sJT] []
GHC.Integer.Type.mkInteger GHC.Types.True sat_sJT;
```

Moreover, determining how much space to allocate for the resulting `Integer`

is a bit work as it for one, you need to traverse the list twice, and then you can't simply derive the exact allocation amount by the list-length alone due to the odd 31-bit chunks.

Instead a more "natural" `mkInteger`

would be desirable, possibly in the style of `unpackCString#`

, in terms of a packed/unboxed vector of machine-word-sized chunks. A better `mkInteger`

could then take a `ByteArray#`

or a `Addr#`

+ length instead, e.g.

```
mkInteger :: Int# -- signum(n) = sign of encoded Integer
-- abs(n) = number of machine-word sized chunks
-> Addr# -- pointer to start of machine-word sized chunks,
-- least-significant chunk first
-> Integer
```