feat(tvix): set up cargo rust project

First steps for baba

Change-Id: Id6a68c5630cb85f280f4dcc7b2acf10c02454fd6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4732
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/src/bin/nix-store.rs

@@ -0,0 +1,3 @@
+fn main () {
+  println!("hello, nix");
+}

tvix/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, tvix!");
+}