rust: add Cargo workspace for abstractions

> A workspace is a collection of one or more packages
https://doc.rust-lang.org/cargo/reference/workspaces.html#workspaces

Rust abstractions of VLC Core will live in src/rust/, while the Rust
modules will live in the respective directories in modules/.

src/rust/Cargo.toml

@@ -0,0 +1,2 @@
+[workspace]
+members = []