Move tokenizer config onto grammar, rename flag

Based on the feedback in
https://github.com/python/black/pull/845#issuecomment-490622711

- Remove TokenizerConfig, and add a field to Grammar instead.
- Pass the Grammar to the tokenizer.
- Rename `ASYNC_IS_RESERVED_KEYWORD` to `ASYNC_KEYWORDS` and
  `ASYNC_IS_VALID_IDENTIFIER` to `ASYNC_IDENTIFIERS`.