Allow setting corpus and crashes dirs in cargo bolero test

[Ekleog]

Fixes #84

[Ekleog]

As drive-by fixes :

• Make libfuzzer also consider all artifacts as corpus items, to be scanned on startup
• Bump rust-toolchain to 1.57 because the tests initially failed in CI, due to a dependency requiring at least 1.57

[Ekleog]

Hmm the remaining errors seem probably unrelated to this change, but I'm not sure how to fix them — I can say that with my real-world test targets things seem to work properly on my machine.

(Oh and obviously cargo test passes here)

[Ekleog]

Probably bumping the MSRV further would fix things, as stable tests pass properly in CI? But I'm not sure that's something you're willing to do, so I'm not adding that to the PR right now :)

[camshaft]

The MSRV policy is a rolling 6+ month window so I'll need to figure out another fix for that. But the corpus override seems reasonable to me.

[Ekleog]

Hmm ok thanks!

What's weird is I think I had already hit this exact issue with cargo-fuzz when the version of cargo-fuzz and the version of rustc didn't have the same LLVM version, but I had seemed to notice that cargo-bolero, for a reason I'm not sure of, didn't hit this issue. My guess had been that cargo-bolero did LLVM version autodetection, but I'm not sure if it's actually true given I'm seeing this now :/

[Ekleog]

AFAICT rust 1.57.0 was released more than 6 months ago (Dec 2, 2021), so hopefully this would be good to go? :)

[camshaft]

It should be now!