Revision 1183638

AVRO-922. C: Value comparison function

This patch adds the avro_value_cmp and avro_value_cmp_fast functions,
which compare two avro_value_t instances using the sort order from the
Avro specification.  Like the avro_value_equal and avro_value_copy
methods, we don't require the two values to have the same value
implementation, but they do need to be instances of the same schema.