Consider adding multi-threading support for Dredd

[JamesLee-Jones]

Currently Dredd can only run on a single thread. This is fine for smaller applications, but when mutating large applications (such as tint), this can take tens of minutes. Adding file level multi-threading to Dredd would allow for a substantial performance increase when mutating large projects.

[afd]

I am not convinced we want to go here. I think there are optimisations that could be done to make Dredd a bit faster, but the bottleneck of a mutation testing campaign isn't doing the one-off mutation. Rather, it is killing all the mutants.