The current structure makes it hard to tell that there are just four
distinct code paths, depending on how many items there are in `bb_items`
and `next_items`. This commit introduces a `match` that clarifies
things.
There are three `Option` fields in `MatcherPos` that are only used in
tandem. This commit combines them, making the code slightly easier to
read. (It also makes clear that the `sep` field arguably should have
been `Option<Option<Token>>`!)
To avoid the strange style where comments force `else` onto its own
line.
The commit also removes several else-after-return constructs, which can
be hard to read.
Currently, we only point at the span of the macro argument. When the
macro call is itself generated by another macro, this can make it
difficult or impossible to determine which macro is responsible for
producing the error.
