feat(tvix/eval): compile simple let ... in ... expressions

These expressions now leave the binding values on the stack, and clean
up the scope after the body of the expression.

While variable access is not yet implemented (as the identifier node
remains unhandled), this already gives us the correct stack behaviour.

Change-Id: I138c20ace9c64502c94b2c0f99a6077cd912c00d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6188
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/vm.rs

@@ -240,6 +240,20 @@ impl VM {
                         });
                     }
                 }
+
+                // Remove the given number of elements from the stack,
+                // but retain the top value.
+                OpCode::OpCloseScope(count) => {
+                    // Immediately move the top value into the right
+                    // position.
+                    let target_idx = self.stack.len() - 1 - count;
+                    self.stack[target_idx] = self.pop();
+
+                    // Then drop the remaining values.
+                    for _ in 0..(count - 1) {
+                        self.pop();
+                    }
+                }
             }
 
             if self.ip == self.chunk.code.len() {