Support the latest haskell-src-exts

[gelisam]

That library has changed again, but this time it didn't break anything, it only added a few extra constructors:

Pattern match(es) are non-exhaustive
In an equation for ‘location’:
    Patterns not matched:
        ClosedTypeFamDecl _ _ _ _ _
        MinimalPragma _ _

I'm not fixing it right now because there is also the extra work of figuring out which new version bounds we need to put on our haskell-src-exts dependency.

[gelisam]

Since this seems to change relatively often, maybe we could find another way of extracting the line number? Maybe by creating an error string incorporating the location, and parsing that?

[gelisam]

I see that I have added an upper bound on haskell-src-exts to hackage a while ago, but didn't actually change the .cabal file in HEAD. Doing this now.

[gelisam]

Wait, no, I'm confusing two separate issues here. The upper bound has been added to address #123, and HEAD has been adjusted to account for it, but requesting the latest version of haskell-src-exts. This issue is about a less important concern: a warning, not a compilation error.

[gelisam]

Closed in favor of the more permanent solution proposed in #149.