chore(tvix/eval): bootstrap some evaluator boilerplate

Change-Id: I7770a20948d18a8506c2418dea21202aa21a6ddc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6064
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/eval.rs

@@ -0,0 +1,5 @@
+use crate::errors::{Error, EvalResult};
+
+pub fn interpret(code: String) -> EvalResult<()> {
+    Ok(())
+}