Implement a flawed version of RenderBlogStreamEntry::new.

There are some shortcomings in this implementation listed in the comments, but this is the general structure.

src/context/blog_stream.rs

@@ -93,6 +93,33 @@ impl RenderBlogStreamEntry {
         output_file: &Path,
         original: &BlogPost,
     ) -> Result<RenderBlogStreamEntry, CustomError> {
-        todo!()
+        // TODO: This link is probably wrong.
+        let link_to_blog_post = get_web_path(
+            config,
+            output_directory,
+            output_file,
+            output_file.strip_prefix(output_directory)?,
+        )?;
+
+        // TODO: Should I guess an index page instead of erroring out?
+        let index_page = original
+            .get_index_page()
+            .ok_or_else(|| format!("Blog post {} needs an index page.", original.id))?;
+
+        let title = index_page.title.clone();
+
+        // TODO: Handle footnotes.
+        let children = index_page
+            .children
+            .iter()
+            .map(|child| RenderDocumentElement::new(config, output_directory, output_file, child))
+            .collect::<Result<Vec<_>, _>>()?;
+
+        Ok(RenderBlogStreamEntry {
+            title,
+            self_link: Some(link_to_blog_post),
+            children,
+            footnotes: Vec::new(),
+        })
     }
 }

src/intermediate/blog_post.rs

@@ -103,7 +103,7 @@ impl BlogPost {
     }
 
     /// Get the blog post page for index.org
-    fn get_index_page(&self) -> Option<&BlogPostPage> {
+    pub(crate) fn get_index_page(&self) -> Option<&BlogPostPage> {
         self.pages
             .iter()
             .find(|page| page.path == Path::new("index.org"))