feat(tvix/eval): implement unary negation operator

Change-Id: I5d012cc073e55d79d7b34b88283aab3164864293
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6075
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/opcode.rs

@@ -17,9 +17,13 @@ pub enum OpCode {
     OpTrue,
     OpFalse,
 
-    // Simple binary operators
+    // Arithmetic binary operators
     OpAdd,
     OpSub,
     OpMul,
     OpDiv,
+
+    // Unary operators
+    OpInvert,
+    OpNegate,
 }