Dictionary array handling

[Jefffrey]

Spun off from discussion in #68

Find way to preserve dictionary encoding in DictionaryArray without having to cast to StringArray as introduced here:

datafusion-orc/src/arrow_reader/decoder/string.rs

Lines 180 to 183 in bb885c0

	// Cast back to StringArray to ensure all stripes have consistent datatype
	// TODO: Is there anyway to preserve the dictionary encoding?
	// This costs performance.
	let array = cast(&array, &DataType::Utf8).context(ArrowSnafu)?;

Some reference from similar parquet issue: apache/arrow-rs#171