feat(tvix/eval): add initial stack-based VM

This can't do anything other than compute a single literal, for now

Change-Id: Ia28f9da51c906b590a198e77a4ca5d45a871106b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6071
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/main.rs

@@ -10,6 +10,7 @@ mod errors;
 mod eval;
 mod opcode;
 mod value;
+mod vm;
 
 fn main() {
     let mut args = env::args();