Avro Compatibility

Decide whether two Avro schemas are compatible, in both directions, under the resolution rules rather than by structural comparison. Backward is a reader on the new schema reading data already written with the old one; forward is a reader still on the old schema reading data written with the new one. Fields match by name with reader aliases consulted, int promotes to long, float and double one way only, and an enum symbol the reader lacks needs an enum default.