With the performance issues with regex figured out and fixed in previous
patches and regex newly gaining support for empty alternations, there is no
reason to keep re2 around anymore. It's only *marginally* faster at creating
the regex which saves at most a couple of ms, but gets beaten by regex in
every other aspect.
This removes the Rust/C/C++ bridge (hooray!), the with-re2 feature, the
conditional code that goes with it, the documentation and relevant part of the