Match Statements

• Make use of match statements in output source from Python 3.10.
• Use new assign tokens: +=, -=, *=, /=, ^=, <<=, >>=
• Change not equal token: /= -> !=
• Add check that self fields can only be used if assigned to first.
• Improve readability of some error messages.
• Start annotating output, though currently annotations are inconsistent.
• Checker can deal with chains of calls a.b.c....
• Simplify grammar such that classes, type definitions, and imports can be placed in any block.
  The checker does not yet have logic to deal with such situations, however.