This is sometimes the best tool for the job. To try and make it less of
a footgun that peek_token() is identical to next_token(), make the
offset required, and note the shared behaviour. Also move them
together, and make the internal code match.
In some situations we want to examine a sequence of tokens that have
been already consumed from a TokenStream. These methods let us do so:
- current_index() provides the current internal token index
- tokens_since() provides a span from the given index to the current one
