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/vm.rs

@@ -254,6 +254,11 @@ impl VM {
                         self.pop();
                     }
                 }
+
+                OpCode::OpGetLocal(local_idx) => {
+                    let value = self.stack[local_idx].clone();
+                    self.push(value)
+                }
             }
 
             if self.ip == self.chunk.code.len() {