Shard across multiple VMs

[sourcefrog]

For CI builds and similar situations it would be nice to have an option --shard k/n that tests a subset of mutants. You could then, for example, launch 8 workers which each test 1/8th of the mutants in separate VMs, for example using GitHub Actions matrices. The tree is clean if they all succeed.

Compared to -j:

• This is only useful if you have multiple machines to run on, so won't help on a laptop.
• It would avoid the issues where high -j can cause RAM exhaustion or thrashing when all the builds or tests spike at the same time.
• You'd get multiple independent outputs, but CI systems should handle the UI of showing which ones failed, and treating failure of any part as an overall failure.