Avoid using unsafe numeric conversions fromInteger and fromIntegral

[brianhuffman]

Haskell functions fromInteger and fromIntegral are used in lots of places in the cryptol codebase. These functions are likely to cause bugs due to their wrap-around behavior when converting a larger type to a smaller one, like Integer to Int. (For example, see #636.)

We should avoid using these functions whenever possible. (I think this would be a good rule to put in our coding style guide, once we have one. Also if we're running hlint along with our automated builds, we could add rules about avoiding fromIntegral and fromInteger.)

Instead of fromInteger or fromIntegral, we should either use:

• a wrapper function whose name explicitly highlights the wraparound behavior, for those cases when that's actually the behavior we want
• a helper function like maybeFromInteger :: Integer -> Maybe Int that explicitly checks for overflow, for those cases where we don't want to wrap around

If there are some situations where fromInteger or fromIntegral really can't be avoided, we should at the very least add comments to each occurrence, explaining why it's OK in that particular situation.

[atomb]

We've fixed a bunch of these, but there are still some remaining. How many do we want to commit to removing before the 2.9 release?

[yav]

I think this is more aspirational and shouldn't be a requirement for the 2.9 release

[brianhuffman]

Once we add these safe numeric conversion functions, I think that will be sufficient to close this ticket.