This update brings with it many bug fixes:
* Better error messages are printed overall. We also include
explicit call out for unsupported features like backreferences
and look-around.
* Regexes like `\s*{` no longer emit incomprehensible errors.
* Unicode escape sequences, such as `\u{..}` are now supported.
For the most part, this upgrade was done in a straight-forward way. We
resist the urge to refactor the `grep` crate, in anticipation of it
being rewritten anyway.
Note that we removed the `--fixed-strings` suggestion whenever a regex
syntax error occurs. In practice, I've found that it results in a lot of
false positives, and I believe that its use is not as paramount now that
regex parse errors are much more readable.
Closes#268, Closes#395, Closes#702, Closes#853
