Implement statistics reporting tool

[MattWindsor91]

Effectively the same as c4t-analyse currently does for analyses (though I want to split the analysis and report stage eventually), but for stats.json.

Initially we need only one mode for this (dump CSV of mutations), but eventually it would be nice to have the following commands:

• human-readable mutation report;
• subject reports per compiler (eg 'bam: 384 cycles, 20 ok, 4 run timeout');
• reset statistics (eg, wipe stats.json).

[MattWindsor91]

The CSV mutation dump is done, as is the outside binary shell, but holding open for the other features.