Automatically merge results from multiple sharded jobs

[sourcefrog]

--shards lets you split mutation testing across multiple CI workers, which is good for latency, but has the downside that you then need to look at multiple log outputs or mutants.out files.

It would be nice to have some tooling that merges them and

• shows in log output everything that was missed
• maybe makes all the mutants output available in one tarball

This does not necessarily have to be built in to the cargo-mutants binary, but it could be.

Perhaps if we have an action to run tests (#141) this could be built in.