build-sys: Add toplevel workspace Cargo.toml

rust-analyzer is happier with this because it understands
the project structure out of the box.

We aren't actually again adding a dependency on Rust/cargo in the core,
this is only used to make `cargo build` work out of the box to build
the Rust test code.

tests/inst/.gitignore

@@ -1,2 +0,0 @@
-target/
-Cargo.lock