Add a field type for storing parsing defects

[jwodder]

Post #52:

Add a DefectsField field type for collecting errors raised during parsing and decoding

• By default, errors are stored as a dict that maps header field names to lists of exceptions

  • What should happen for scanner and body errors?
    • Idea: Don't catch scanner errors … for now
    • Idea: Store them in the dict with the key set to a SCANNING or BODY enum or token
    • Idea: Wrap all decoder errors in custom DecoderError instances
      • Subclasses:
        • FieldDecoderError(post-alias-name, value, error)
        • ExtraFieldsDecoderError(value, error)
        • BodyDecoderError(value, error)

• Non-extra fields can now take a required: bool parameter so that lack of a required field can be caught & registered as a defect

• Errors are stored after calling .with_traceback(None) on them and their chain of causes (__cause__) & contexts (__context__) in order to reduce memory use

• Should defects mode be toggleable by an option when parse*() is called?