- Also fixes double curly braces when printing HByte structs when len is 0

Add tests for .set_top_node(0, None), and .set_top_node(None, None)

Also ensure cleanup runs in HParseResult test

- ASTManager helper class to hold the top node of the ast to be printed
- HDoParseRetBreakpoint
- ast printing commands
- Update BreakpointManager tests with HDoParseRetBreakpoint tests
- Add support in TopLevelParse to select a stack item for printing the AST node of