Add differential fuzzing between different regex engines

[addisoncrump]

We observed some success with differential fuzzing between the regex 1.8 and 1.9 in #848. The 1.9 release promises to have many different engines with different optimisation strategies; it would be good to take advantage of this to verify the engines with each other, which may identify subtle bugs with the compilation or execution of regexes.

This is discussed in some detail (with a proof of concept) in #978 but requires quite a bit more engineering, and can only really be written by someone intimately familiar with the 1.9 release as it would need to account for all potential use cases of the crate.