Mitigating the size impact of the "regex" dependency

[talagrand]

When doing binary size analysis of my project, I counted that "regex" and its dependencies were 3x the binary size impact of boon itself. Suggestion: it would be nice to make a "regex" crate feature that's part of default-features, and allow clients to opt out of using it. Alternatively, or in addition, it would be valuable to allow the option to switch to the regex_lite crate.