Tracking issue: crater is slow

[aidanhs]

Crater has a number of inefficiencies that can be resolved to make life better for everyone.

Impactful

• prepare-ex command has extreme latency due to null registry updates #96 - generate-lockfile does a registry update
• Reconsider running generate-lockfiles in docker #101 - reconsider doing generate-lockfile inside a docker container
• Parallelise prepare-local
• Parallelise run-tc (currently relies on inherent parallelism of cargo and builds everything into a huge target directory - some complexity here to figure how to manage the shared resources correctly).
• Investigate using codegen-units, experiments show it speeds up rustc build a lot, but slows down tests. ThinLTO may help also.

Nice-to-have

• Consider offering a build-only mode (probably build tests as well)

Metrics

• Monitor which crates are taking the longest and what the long tail of slow crates looks like
• Get a breakdown of what phases are taking time (e.g. build, test build, testing) to inform future work

Speculative

• allow cargobomb to distribute over multiple machines

[pietroalbini]

Parallelization of prepare-local and run-tc is now implemented with run-graph --threads N, and build-only crater runs are possible with --mode build-only.

[Mark-Simulacrum]

codegen-units are "implemented" IMO due to default support within rustc (i.e., we run with codegen-units at 16 already).