Single row mode streaming

[fehrenbach]

This adds a version of foldWith that uses single row mode instead of a cursor under the hood.

The main benefit is that queries show up as themselves in logs and pg_stat_statements and the like (c.f., #100).

This PR doesn't yet define foldSingleRowMode and foldSingleRowMode_ (the versions using FromRow rather than an explicit parser). I'd be happy to add them or instead add a SingleRowMode constructor to FoldOptions. (Mild preference for the extra functions.)

[fehrenbach]

It turns out that this is not really what I want. Ultimately I want to produce a servant stream, using its SourceT / StepT types. This means I need a right fold, not a left fold.

Right folds are awkward as an API because there is no guarantee that the combining function actually consumes all of the results. Not consuming all results means the connection is left in a bad state where the next query will complain that another command is already in progress. (This isn't that different from trying to execute another query in the combining function of a left fold, but it does seem worse somehow.)

If anyone has any clever ideas, please let me know.