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/assign_to_closure.wren

@@ -0,0 +1,25 @@
+var f = null
+var g = null
+
+{
+  var local = "local"
+  f = fn {
+    io.write(local)
+    local = "after f"
+    io.write(local)
+  }
+
+  g = fn {
+    io.write(local)
+    local = "after g"
+    io.write(local)
+  }
+}
+
+f.call
+// expect: local
+// expect: after f
+
+g.call
+// expect: after f
+// expect: after g