Add MemberGroup struct for groups of members items

Previosuly, we stored the members as a vector of vectors, group by a
string -- their title.  This was very hard to read and also too simple:
For example, there can be multiple blocks of methods in the
documentation for a struct.  Therefore we introduce a new type,
MemberGroup, that represents a group of member items with an optional
title.

src/doc.rs

@@ -71,7 +71,13 @@ pub struct Doc {
     pub ty: ItemType,
     pub description: Option<String>,
     pub definition: Option<String>,
-    pub members: Vec<(String, Vec<Doc>)>,
+    pub groups: Vec<(ItemType, Vec<MemberGroup>)>,
+}
+
+#[derive(Clone, Debug)]
+pub struct MemberGroup {
+    pub title: Option<String>,
+    pub members: Vec<Doc>,
 }
 
 impl Name {
@@ -425,7 +431,16 @@ impl Doc {
             ty,
             description: Default::default(),
             definition: Default::default(),
-            members: Default::default(),
+            groups: Default::default(),
+        }
+    }
+}
+
+impl MemberGroup {
+    pub fn new() -> Self {
+        MemberGroup {
+            title: None,
+            members: Vec::new(),
         }
     }
 }

src/parser.rs

@@ -110,10 +110,10 @@ pub fn parse_module_doc(item: &doc::Item) -> anyhow::Result<doc::Doc> {
     let mut doc = doc::Doc::new(item.name.clone(), item.ty);
     doc.description = description.map(|n| get_html(n.as_node())).transpose()?;
     for item_type in MODULE_MEMBER_TYPES {
-        let members = get_members(&document, item, *item_type)?;
-        if !members.is_empty() {
-            doc.members
-                .push((item_type.group_name().to_string(), members));
+        let mut group = doc::MemberGroup::new();
+        group.members = get_members(&document, item, *item_type)?;
+        if !group.members.is_empty() {
+            doc.groups.push((*item_type, vec![group]));
         }
     }
     Ok(doc)

src/viewer/text/mod.rs

@@ -33,11 +33,17 @@ impl<P: Printer> TextViewer<P> {
             .print_heading(1, &format!("{} {}", doc.ty.name(), doc.name.as_ref()))?;
         self.print_opt(doc.definition.as_deref())?;
         self.print_opt(doc.description.as_deref())?;
-        for (heading, items) in &doc.members {
-            if !items.is_empty() {
-                self.printer.println()?;
-                self.printer.print_heading(2, heading)?;
-                self.print_list(items.iter().map(|i| {
+        for (ty, groups) in &doc.groups {
+            self.printer.println()?;
+            self.printer.print_heading(2, ty.group_name())?;
+
+            for group in groups {
+                if let Some(title) = &group.title {
+                    self.printer.println()?;
+                    self.printer.print_heading(3, title)?;
+                }
+
+                self.print_list(group.members.iter().map(|i| {
                     if let Some(description) = &i.description {
                         format!("{}<br/>{}", i.name.last(), description)
                     } else {