avro

AVRO-919. C: Produce JSON encoding for Avro values

This patch adds an avro_value_to_json function, which produces the JSON
encoding on an Avro value.  This is a pretty basic translation of the
existing avro_datum_to_json function, which has been modified to use the
new value API.  The avro_datum_to_json function is now implemented using
the datum type's value wrapper.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1183634 13f79535-47bb-0310-9956-ffa450edef68