feat(tvix/eval): add local identifier access

This makes basic `let ... in ...` statements work correctly. It does
not yet account for the call frames pushed into the VM during function
application.

Change-Id: I67155171daf1a43011b96716dd9d1ab04b27db33
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6190
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -29,6 +29,9 @@ pub enum Error {
 
     // Dynamic keys are not allowed in let.
     DynamicKeyInLet(rnix::SyntaxNode),
+
+    // Unknown variable in statically known scope.
+    UnknownStaticVariable(rnix::types::Ident),
 }
 
 impl Display for Error {