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/use_global_in_method.wren

@@ -0,0 +1,15 @@
+var global = "global"
+// TODO(bob): Forward reference to global declared after use.
+
+class Foo {
+  method {
+    io.write(global)
+  }
+
+  static classMethod {
+    io.write(global)
+  }
+}
+
+Foo.new.method // expect: global
+Foo.classMethod // expect: global