Implement mutation testing

[MattWindsor91]

This is the main issue tracking implementing just enough mutation testing support to be able to run campaigns against my run-time mutated form of llvm. Parts of the support might be useful for other forms of mutation testing eg. compile-time.

Once we've decided whether to do global mutation accounting or per-machine mutation accounting, I'll update this issue with more details. I'll also spin out separate issues for these throughout the morning.

• Support passing environment variables to compilers
• Add mutation test simulation to gccn't (Add mutation test simulation to gccn't #103)
• Add configuration language for specifying mutation ranges (Add mutation range specification to config #105)
• Implement observer for killed and hit mutants (Implement mutation observer #104)
• Implement mutation testing switch in c4t toplevel
• Run testing experiments with gccn't
• Run testing experiments with mutated LLVM

[MattWindsor91]

Most of the new work will go in a mutation package, I suspect, to try keep things clean. This should depend on the relevant director bits and be injected through the app, ideally.