Remove the bogus special case from `Parser::look_ahead`.

The general case at the bottom of `look_ahead` is slow, because it
clones the token cursor. Above it there is a special case for
performance that is hit most of the time and avoids the cloning.
Unfortunately, its behaviour differs from the general case in two ways.

- When within a pair of delimiters, if you look any distance past the
  closing delimiter you get the closing delimiter instead of what comes
  after the closing delimiter.

- It uses `tree_cursor.look_ahead(dist - 1)` which totally confuses
  tokens with token trees. This means that only the first token in a
  token tree will be seen. E.g. in a sequence like `{ a }` the `a` and
  `}` will be skipped over. Bad!

It's likely that these differences weren't noticed before now because
the use of `look_ahead` in the parser is limited to small distances and
relatively few contexts.

Removing the special case causes slowdowns up of to 2% on a range of
benchmarks. The next commit will add a new, correct special case to
regain that lost performance.

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