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/field/multiple.wren

@@ -27,3 +27,4 @@ foo.write
 // TODO(bob): Inherited fields.
 // TODO(bob): Trying to get or set a field outside of a class.
 // TODO(bob): Fields in nested classes.
+// TODO(bob): Closing over fields.