Is your feature request related to a problem or challenge?

DataFusion uses ColumnarValue to represent Arrays and Scalars.

#[derive(Clone, Debug)]
pub enum ColumnarValue {
    /// Array of values
    Array(ArrayRef),
    /// A single value
    Scalar(ScalarValue),
}

Perhaps we should use this in the UDF signatures. This avoids the need to pass in row counts (see #57).

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