Long awaited update

• Several important refactorings in contexts.ParseContext
• Make ignorecase settings apply to defined @@keywords
• Move checking of keywords used as names into ParseContext
• Output of generated parsers again matches that of model parsers
• Improve "expecting one of:" messages so elements are in declaration order
• Stop code generation if there are closures over possibly empty expressions
• Preserve name declaration order in returned AST
• Update the bootstrap parser (tatsu/bootstrap.py) to the generated parser
• Now generated parser's main() only outputs the JSON for the parse AST
• Minor version bumped in case the many fixes break backwards-compatibility
• Minor documentation issues fixed
• All tests run with Python 3.8, 3.9, 3.10