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/vm.rs

@@ -170,6 +170,17 @@ impl VM {
                         self.globals.insert(name, val);
                     });
                 }
+
+                OpCode::OpGetGlobal(name_idx) => {
+                    let name = self.chunk.constant(*name_idx);
+                    with_type!(self, name, Value::String(name), {
+                        let val = match self.globals.get(name) {
+                            None => unimplemented!("variable not found error"),
+                            Some(val) => val.clone(),
+                        };
+                        self.push(val)
+                    });
+                }
             }
 
             #[cfg(feature = "disassemble")]