v1.0.0-rc.4

• Rename lowest API level more appropriately as a CST parser rather than an AST parser, so you'll now need to import yaml/parse-cst rather than yaml/parse-ast to use it.
• Simplify the object-level API, dropping Node#origTag, skip setting empty values, and letting Pair extend Node.
• Document the YAML.Document contents more clearly as an AST.