Add benchmarks of regex in release mode

[oyvindln]

This adds benchmarks of regex using the --release flag.
I also did some small changes to the documentation as I hit some snags.
Let's see if I did this right..

Fixes #140

[Mark-Simulacrum]

Instead of regex-opt-version-X can you rename the directories to regex-opt-X? And then squash the two regex-related commits together? I think we'll be good to go then.

[oyvindln]

@Mark-Simulacrum Updated as you requested.

[Mark-Simulacrum]

Thanks!

[michaelwoerister]

🎉