Get closures working!

In the process, I had to change the grammar. There is now a strong
separation between statements and expressions. The code was just wrong
before when it popped locals at the end of a block scope because there
could be temporaries on the stack if the block was in expression
position. This fixes that.

Still need to implement closing over `this`.

test/closure/reuse_closure_slot.wren

@@ -0,0 +1,19 @@
+{
+  var f = null
+
+  {
+    var a = "a"
+    f = fn io.write(a)
+  }
+
+  {
+    // Since a is out of scope, the local slot will be reused by b. Make sure
+    // that f still closes over a.
+    var b = "b"
+    f.call // expect: a
+  }
+}
+
+// TODO(bob): Closing over this.
+// TODO(bob): Close over fn/method parameter.
+// TODO(bob): Maximum number of closed-over variables (directly and/or indirect).