feat(tazjin/rlox): Initial bytecode representation

This is significantly simplified from the version in the book, since
I'm using Rust's Vec and not implementing dynamic arrays manually.

We'll see if I run into issues with that ...

Change-Id: Ie3446ac3884b850f3ba73a4b1a6ca14e68054188
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2413
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

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

@@ -0,0 +1,8 @@
+#[derive(Debug)]
+pub enum OpCode {
+    /// Access a constant for use.
+    OpConstant(usize),
+
+    /// Return from the current function.
+    OpReturn,
+}