Add CorpusStorage::find_extra returning the query alternative for each match

[matthias-stemmler]

This adds a CorpusStorage::find_extra method that returns for each match which query alternative produced it. In case multiple alternatives produce the same match, it gets attributed to the first alternative.

The new method has the same signature as CorpusStorage::find, except that it returns a Vec<MatchExtra> instead of a Vec<String>. The MatchExtra struct is marked #[non_exhaustive] so that adding more fields later isn't a breaking change. The names find_extra and MatchExtra follow the convention established for count_extra and CountExtra.

Performance: the alternative index is a plain usize carried alongside the numeric match group, so it stays out of the sort key and the uniqueness check, and match IDs are still resolved to strings at the end. I did extract the common parts of find and find_extra: The added cost for find is that usize per match in the temporary BTree index plus one iterator indirection; it showed no measurable effect on the GUM test corpus.

I didn't extend the FFI surface for now.

Closes #341.