Improve internal type safety, and import GHCi fix from persistent-sqlite

[joeyadams]

This pull request changes a lot internally, but should not cause any external breakage. It's mostly improvements to internal typing. However, the last commit is a hack imported from persistent-sqlite so direct-sqlite can be used with GHCi.

Here are all the user-facing changes I'm aware of:

• Error constructors now have documentation (copied from SQLite documentation).
• direct-sqlite can now be used with GHCi on 64-bit Linux (I haven't tested it anywhere else yet)
• If the SQLite3 API returns an unrecognized result code, it is reported with Prelude.error instead of a non-exhaustive pattern match error.
• direct-sqlite now needs the EmptyDataDecls extension (internally).
• sqlite3.h is bundled, too, meaning the tarball will be slightly bigger.

[nurpax]

Great change.

[IreneKnapp]

This all looks quite solid! I know the feeling of a patch going into the void, so I'm going to do my best to review it rapidly for you, but it is quite a lot to read through. I kinda wish it had been submitted as separate requests for each incremental patch (or even each logical group of them), just as something to keep in mind for the future.