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/constructor/named.wren

@@ -3,7 +3,7 @@ class Foo {
   this baz { io.write("this baz") }
   this bar(arg) { io.write("this bar " + arg) }
 
-  toString { "Foo" }
+  toString { return "Foo" }
 }
 
 // Different names.

test/constructor/new.wren

@@ -4,7 +4,7 @@ class Foo {
   this new(a) { io.write(a) }
   this new(a, b) { io.write(a + b) }
 
-  toString { "Foo" }
+  toString { return "Foo" }
 }
 
 // Can overload by arity.