Avro Schema Audit

Audit one Avro schema for what it makes impossible later. Reports a field with no default, which cannot be removed without breaking readers; a nullable field with no default; a null default where null is not the first union branch, which is invalid; an enum with no default, which makes adding a symbol permanently breaking; an absent namespace on a top-level record; a logical type on a base the specification disallows; and a binary float on a monetary field name.