feat: provide a cargo feature to disable the proc macro inference in #221

[rbtcollins]

Reading Cargo.toml during compilation is problematic - there isn't necessarily a Cargo.toml to read. This was added in #221

For us, we build with Bazel, and do happen to have a Cargo.toml, but it isn't copied into the build environment - because rustc doesn't typically need it. And indeed with about 1000 crates in use, this 0.20 update of rstest is the first we've encountered it.

[la10736]

Ok, I've suspected this issue but l I've undervalued it. I'll add a feature flag for it disabled by default ASAP.

[la10736]

I choose to add the feature crate-name but enabled by default: you can disable this feature with:

rstest = { version = "0.21.0", default-features = false }

The 0.21.0 version will be on-line soon.