Deprecating fromIntegral

[Vanessa McHale]

-1 from me, massive work to overhaul the ecosystem.

Maybe a haddock comment first?

> On Aug 7, 2020, at 10:08 PM, Niklas Hambüchen via Libraries <libraries at haskell.org> wrote:
> 
> Today I found another big bug caused by `fromIntegral`:
> 
>    https://github.com/haskell-crypto/cryptonite/issues/330
> 
> Incorrect hashes for all hash algorithms beyond 4 GiB of input. SHA hash collisions in my productions system.
> 
> Restating what I said there:
> 
> * Until we deprecate fromIntegral, Haskell code will always be subtly wrong and never be secure.
> * If we don't fix this, people will shy away from using Haskell for serious work (or learn it the hard way). Rust and C both do this better.
> * If the authors of key crypto libraries fall for these traps (no blame on them), who can get it right? We should remove the traps.
> 
> The wrong code,
> 
>    hashInternalUpdate ctx d (fromIntegral $ B.length b)
> 
> exists because it simply does not look like wrong code. In contrast,
> 
>    hashInternalUpdate ctx d (fromIntegralWrapping $ B.length b)
> 
> does look like wrong code and would make anyone scrolling by suspicious.
> 
> We can look away while continuing to claim that Haskell is a high-correctness language, or fix stuff like this and make it one.
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