Row contains performance

[tanner0101]

Add performant method for checking if a PostgresRow contains a given column.

[fabianfett]

@gwynne Do you know what we would use this feature for?

[fabianfett]

PostgresRandomAccessRow has this feature today:

struct PostgresRandomAccessRow {
    /// Checks if the row contains a cell for the given column name.
    /// - Parameter column: The column name to check against
    /// - Returns: `true` if the row contains this column, `false` if it does not.
    func contains(_ column: String) -> Bool
}