I got confused and refactored a bit, now the naming should be more
consistent. This is the basis for the range formatting work.
Chages:
* `format_module` -> `format_module_source` (format a string)
* `format_node` -> `format_module_ast` (format a program parsed into an
AST)
* Added `parse_ok_tokens` that takes `Token` instead of `Result<Token>`
* Call the source code `source` consistently
* Added a `tokens_and_ranges` helper
* `python_ast` -> `module` (because that's the type)
## Summary
The tokenizer was split into a forward and a backwards tokenizer. The
backwards tokenizer uses the same names as the forwards ones (e.g.
`next_token`). The backwards tokenizer gets the comment ranges that we
already built to skip comments.
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Co-authored-by: Micha Reiser <micha@reiser.io>
