v6.4.0

• Add a flag --sources-comment which can be set to false to suppress the list of input files in the generated source comment
• Add support for computing the canonical form of schemas and the Avro 64-bit CRC
• Default to null codec when decoding OCF files without a specified codec
• Add Attribute method to AvroType and Definition
• Several fixes to resolving schema evolution to align with the Avro specification