Add Row trait [WIP]

[andygrove]

As a first step towards moving to async and returning impl Stream<Row> this PR aims to introduce a Row trait and change the ResultSet.next() method to return an Option<Row>.

/// Result set from executing a query against a statement
pub trait ResultSet {
    /// get meta data about this result set
    fn meta_data(&self) -> Result<Box<dyn ResultSetMetaData>>;

    /// Fetch the next row
    fn next(&mut self) -> Result<Option<Box<dyn Row>>>;
}

/// Row
pub trait Row {
    fn get_i8(&self, i: u64) -> Result<Option<i8>>;
    fn get_i16(&self, i: u64) -> Result<Option<i16>>;
    fn get_i32(&self, i: u64) -> Result<Option<i32>>;
    fn get_i64(&self, i: u64) -> Result<Option<i64>>;
    fn get_f32(&self, i: u64) -> Result<Option<f32>>;
    fn get_f64(&self, i: u64) -> Result<Option<f64>>;
    fn get_string(&self, i: u64) -> Result<Option<String>>;
    fn get_bytes(&self, i: u64) -> Result<Option<Vec<u8>>>;
    //TODO add accessors for date, time, timestamp and other ANSI SQL types
}

One concern that I have with this approach is that now each Row is boxed so that seems inefficient?

Also, I'm struggling to implement it for Postgres and SQLite due to lifetime issues.