v2.0.0

Full Changelog: v1.6.0...v2.0.0

v2.0.0 Release (stable)

• removed feed_callback()
• fixed ParseError variants
  - InvalidNonTerm is deleted since it never happens. Moved it to unreachable!
  - CallbackError is deleted
• add type aliases for Rule, State, ParseError in generated structs
• add exclamation ! pattern to ignore <RuleType> of token in production rule