Attempt running static analysis checks within the thread running the mutated code

[Ocramius]

Infection supports --threads or -j as parameter: when using this tool together with infection, you can clearly observe how the entire process halts when a mutant is found, and needs to be passed through vimeo/psalm for inspection.

This "stuttering" is because type checking is executed in the main thread, instead of in child threads.

If we manage to bootstrap and run psalm in child threads, we can speed up this process massively, although at the overhead of having to respawn the whole psalm stack at every executed test.

This is very much related to #3, perhaps mutually exclusive with that.