2.0.0 tasks

[braingram]

This is to track changes proposed in https://github.com/asdf-format/asdf-standard/wiki/The-revenge-of-2.0.0-Roadmap

• Remove link between format and core schemas: use new terminology to disambiguate "standard" #473
• Use new terminology use new terminology to disambiguate "standard" #473
• Decouple schemas and format (and drop undocumented ASDF_STANDARD comment)
• Store block index in a block
• Treat offsets as relative on absolute offset matching failure
• Multiple documents per file: Support for multiple yaml document in one ASDF file #504
• Put asdf_library and history in a separate document
• Update to YAML 1.2
• Add checksum to block header
• Drop Integer drop unused arbitrary precision integer from standard #453
• Drop ExternalArrayReference
• Drop JSON Pointer Review of features related to external references #261 and JSON pointer handling of integer mapping key is undefined #477
• Simplify complex schema Review complex number representation #263
• Drop style-related items from metaschema: Drop style-related properties from the metaschema #256
• Remove examples from metaschema: Remove examples from yaml-schema #472 (comment)