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/nested_closure.wren

@@ -0,0 +1,21 @@
+var f = null
+
+fn {
+  var a = "a"
+  fn {
+    var b = "b"
+    fn {
+      var c = "c"
+      f = fn {
+        io.write(a)
+        io.write(b)
+        io.write(c)
+      }
+    }.call
+  }.call
+}.call
+
+f.call
+// expect: a
+// expect: b
+// expect: c