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/closed_closure_in_method.wren

@@ -0,0 +1,14 @@
+var foo = null
+
+{
+  var local = "local"
+  class Foo {
+    method {
+      io.write(local)
+    }
+  }
+
+  foo = Foo.new
+}
+
+foo.method // expect: local