feat(tazjin/rlox): Support trivial literals in bytecode compiler

Adds support for true, false & nil. These each come with a new
separate opcode and are pushed directly on the stack.

Change-Id: I405b5b09496dcf99d514d3411c083e0834377167
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2571
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/bytecode/opcode.rs

@@ -1,8 +1,13 @@
 #[derive(Debug)]
 pub enum OpCode {
-    /// Access a constant for use.
+    /// Push a constant onto the stack.
     OpConstant(usize),
 
+    // Literal pushes
+    OpNil,
+    OpTrue,
+    OpFalse,
+
     /// Return from the current function.
     OpReturn,