Structex - structural regular expressions for Rust

This crate provides a generic structural regular expression engine that can be backed by a user provided regular expression engine. Support for using the regex crate is provided from this crate under the regex feature.

WIP

This crate is still very much a work in progress. Error reporting needs to be improved, testing needs to be completed and public APIs need to be documented. There is also a fair amount of helper functionality that should be added.