Add a `query_one` method that ensures a single result

[tgross35]

This kind of method is helpful for when you expect a single item, and any more would indicate an error. The signature would be approximately:

fn query_one<T, Q>(&mut self, query: Q) -> Result<T>
where
    Q: AsRef<str>,
    T: FromRow,

Any result count other than one returns an error.

[blackbeam]

Hi. Whether some query returns one or multiple rows depends mostly on the query itself that, in turn, depends on the application logic. I believe this shouldn't be a driver problem. From the driver perspective this adds a new error variant ManyOrNoRowsReturnedWhileExactlyOneRowIsExpected that seems odd for a database driver to have.