look-around assertions.
* `pool` provides a way to reuse mutable memory allocated in a thread safe
manner.
* `prefilter` provides APIs for building prefilters and using them in searches.
* `primitives` are what you might use if you're doing lower level work on
automata, such as walking an NFA state graph.
* `syntax` provides some higher level convenience functions for interacting
with the `regex-syntax` crate.
* `wire` is useful if you're working with DFA serialization.