Revision 1057661

AVRO-725. Add avro_schema_get_subschema function.

This new function returns the immediate subschema with the given name.
For records, this is the name of the field.  For unions, it's the name
of the branch's schema type.  For arrays and maps, there is a single
special name (respectively "[]" and "{}") that represents the item or
value subschema.