Implement mutation observer

[MattWindsor91]

As part of #102, we need an observer that hooks into analyses*, scours compiler logs, and does the following for each compilation:

• If we detect the words MUTATION HIT: in a log line, parse an integer and increment a hit counter for that mutant;
• If we do so and the compilation was deemed interesting, declare that mutant killed by that compilation and send a signal upstream (ie to whatever is controlling the mutant time-slicing).

In both situations, we would probably also have a log file open and dump information about the hit/kill.

I'm not yet sure whether this would happen synchronously as part of the call into the observer, or involve a channel of some sort.

• It hasn't been decided yet whether it hooks into these at the machine level or global level, and so whether this would be an analysis or a cycle-analysis.

[MattWindsor91]

• Implement and test line parsing
• Implement (and test?) per-compilation behaviour
• Implement (and test?) observer boilerplate

[MattWindsor91]

This is probably, actually, best done as an analysis pass, followed by an observer that scrapes the analyser output. I might just do that.

[MattWindsor91]

Done (albeit somewhat hardcoded in for now).