Support generating witnesses for manifest lints

[obi1kenobi]

Lints' witness field currently only supports creating witnesses for source code breakage, not manifest breakage. It can generate Rust source code witnesses, but proving that manifest breakage has occurred requires generating a Cargo.toml file (plus, presumably, an empty lib.rs).

We need a way to optionally specify a Cargo.toml template in addition to the source code template that is currently supported by the witness field in lint files.

cc @suaviloquence — this is low priority at the moment, just a heads-up so you can keep an eye on it and mull over how we'd design witness support for manifests.

[suaviloquence]

Would the lib.rs always necessarily be empty?

[obi1kenobi]

I'm not sure. Off the top of my head, I can't think of an example where we'd need lib.rs contents to prove breakage, but then again I can only think of a handful of manifest lints at the moment anyway.