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/method/static.wren

@@ -1,9 +1,9 @@
 class Foo {
-  bar { "on instance" }
-  static bar { "on metaclass" }
+  bar { return "on instance" }
+  static bar { return "on metaclass" }
 
-  bar(arg) { "on instance " + arg }
-  static bar(arg) { "on metaclass " + arg }
+  bar(arg) { return "on instance " + arg }
+  static bar(arg) { return "on metaclass " + arg }
 }
 
 io.write("on metaclass " + "arg") // expect: on metaclass arg