feat(tazjin/rlox): Implement global variable access

This also includes a fix for an issue where the identifiers of
variables were pushed onto the stack, which is incorrect.

Change-Id: Id89b388268efad295f29978d767aa4b33c4ded14
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2594
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

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

@@ -108,3 +108,13 @@ fn strings() {
     expect_str("\"hello\";", "hello");
     expect_str("\"hello\" + \" world\";", "hello world");
 }
+
+#[test]
+fn variables() {
+    expect_num("var a = 5; a;", 5.0);
+    expect_num("var a = 5; var b = 2; a * b;", 10.0);
+    expect_str(
+        "var greeting = \"hello\"; var name = \"Zubnog\"; greeting + \" \" + name;",
+        "hello Zubnog",
+    );
+}