Expose statements as first-class entities.

Currently, we are able to interleave IO with fetching data from SQLite via fold/fold_. However, the type signature of the callback is constraining. This does not play well with iteratees libraries like pipes, which are monad transformers. If we want to support this use case, we need to expose statements as first-class entities.

This is loosely related to issue #14, but alternatively I can open a different issue.

For good measure, I have added methods to open and close statements, as this allows for the integration of sqlite-simple with a resource finalization library like ResourceT.