avro

AVRO-2999: Optimize Ruby union encoding (#1037)

* AVRO-2999: Optimize Ruby union encoding

This optimizes several bottlenecks when encoding unions in the Avro Ruby
library.

Note: Encoding unions is still expensive because the code must
determine which member of the union a datum is targeting. Allowing
clients to explicitly specify this would speed up serialization even
further but that requires a larger API change.