posts: move files into date-prefixed directories

Using date prefixes is better for sorting articles at a glance. Using
directories is more uniform if and when I need to add files to go with
an article.

content/posts/2020-07-22-polymorphic-flyweight-cpp/index.md

@@ -13,8 +13,8 @@ favorite: false
 ---
 
 Coming back from our last post about [generic flyweights in C++]({{< relref
-"generic-flyweight-cpp.md" >}}), we can write a flyweight that can be used with
-any abstract base classes.
+"../2020-07-16-generic-flyweight-cpp/index.md" >}}), we can write a flyweight
+that can be used with any abstract base classes.
 
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