If we do, this results in extracting `foofoofoo` from `(\wfoo){3}`,
which is wrong. This does prevent us from extracting `foofoofoo` from
`foo{3}`, which is unfortunate, but we miss plenty of other stuff too.
Literal extracting needs a good rethink (all the way down into the regex
engine).
Fixes#93
In particular, if we had an inner literal and were doing a case insensitive
search, then the literals are dropped because we previously only allowed
a single inner literal to have an effect. Now we allow alternations of
inner literals, but still don't quite take full advantage.
