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/this/this_in_method.wren

@@ -1,6 +1,6 @@
 class Foo {
-  bar { this }
-  baz { "baz" }
+  bar { return this }
+  baz { return "baz" }
 }
 
-Foo.new.bar.baz
+io.write(Foo.new.bar.baz) // expect: baz