Run individual benchmarks in separate sub-processes

[psalz]

To increase resilience against hard-crashing benchmarks, this change causes the framework to no longer directly execute the individual benchmarks. Instead, for each call to BenchmarkApp::run(), a subprocess is spawned, which, based on an environment variable, decides what benchmark to execute.

[psalz]

Okay so this definitely needs a signal handler or something to kill the parent process :D