Squash some more warnings from unused member in Machine

We're not using objects yet, so the object arena isn't being used.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/machine.rs

@@ -67,7 +67,7 @@ pub struct Machine {
     globals: BTreeMap<Word, Value>,
     stack: Vec<Value>,
     max_stack_size: Option<usize>,
-    arena: Rc<RefCell<Arena>>,
+    _arena: Rc<RefCell<Arena>>,
     quote_table: QuoteTable,
     scope_stack: ScopeStack,
     call_stack: Vec<Frame>,
@@ -84,7 +84,7 @@ impl Machine {
             globals,
             stack: Default::default(),
             max_stack_size,
-            arena: Rc::new(RefCell::new(arena)),
+            _arena: Rc::new(RefCell::new(arena)),
             quote_table: Default::default(),
             scope_stack,
             call_stack: Default::default(),