Dictionary array handling

[Jefffrey]

Spun off from discussion in datafusion-contrib/datafusion-orc#68

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

https://github.com/datafusion-contrib/datafusion-orc/blob/bb885c03cfedb4d4e16d0203b90a9789463e2fb7/src/arrow_reader/decoder/string.rs#L180-L183

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