Anchor and UserAnchor nodes can contain children

The impetus for this change was to help make the MarkupConverter code
more robust. However, it's also possible that an Anchor can contain
styled text. For example, in markdown someone might write a link that
contains some <strong> text:

```markdown
[this link is so **good**](https://example.com)
```

This setup will now allow that. Unknown if UserAnchor can ever contain
any text that isn't just the user's name, but it's easy to deal with
and makes the typing much easier.

spec/classes/markup_converter_spec.cr

@@ -126,7 +126,7 @@ describe MarkupConverter do
 
     result.should eq([
       Text.new("I am a "),
-      Anchor.new(text: "Link", href: "https://example.com"),
+      Anchor.new(children: [Text.new("Link")] of Child, href: "https://example.com"),
     ])
   end
 
@@ -160,7 +160,7 @@ describe MarkupConverter do
 
     result.should eq([
       Text.new("Hi "),
-      UserAnchor.new(text: "Dr Nick", userId: "abc123"),
+      UserAnchor.new(children: [Text.new("Dr Nick")] of Child, userId: "abc123"),
       Text.new("!"),
     ])
   end

spec/classes/paragraph_converter_spec.cr

@@ -190,7 +190,10 @@ describe ParagraphConverter do
         Image.new(src: "image.png", originalWidth: 1000, originalHeight: 600),
         FigureCaption.new(children: [
           Text.new("Image by "),
-          Anchor.new(href: "https://unsplash.com/@someuser", text: "someuser"),
+          Anchor.new(
+            children: [Text.new("someuser")] of Child,
+            href: "https://unsplash.com/@someuser"
+          ),
         ] of Child),
       ] of Child),
     ]

spec/components/page_content_spec.cr

@@ -40,7 +40,7 @@ describe PageContent do
 
   it "renders an anchor" do
     page = Page.new(nodes: [
-      Anchor.new(href: "https://example.com", text: "link"),
+      Anchor.new(children: [Text.new("link")] of Child, href: "https://example.com"),
     ] of Child)
 
     html = PageContent.new(page: page).render_to_string
@@ -229,7 +229,7 @@ describe PageContent do
 
   it "renders a user anchor" do
     page = Page.new(nodes: [
-      UserAnchor.new(userId: "abc123", text: "Some User"),
+      UserAnchor.new(children: [Text.new("Some User")] of Child, userId: "abc123"),
     ] of Child)
 
     html = PageContent.new(page: page).render_to_string

src/classes/markup_converter.cr

@@ -39,9 +39,9 @@ class MarkupConverter
         case markup.type
         when PostResponse::MarkupType::A
           if href = markup.href
-            container = Anchor.new(href: href, text: to_be_marked)
+            container = Anchor.new(children: [Text.new(to_be_marked)] of Child, href: href)
           elsif userId = markup.userId
-            container = UserAnchor.new(userId: userId, text: to_be_marked)
+            container = UserAnchor.new(children: [Text.new(to_be_marked)] of Child, userId: userId)
           else
             container = Empty.new
           end

src/components/page_content.cr

@@ -13,7 +13,7 @@ class PageContent < BaseComponent
   end
 
   def render_child(node : Anchor)
-    a(href: node.href) { text node.text }
+    a(href: node.href) { render_children(node.children) }
   end
 
   def render_child(node : BlockQuote)
@@ -95,6 +95,6 @@ class PageContent < BaseComponent
   end
 
   def render_child(node : UserAnchor)
-    a(href: node.href) { text node.text }
+    a(href: node.href) { render_children(node.children) }
   end
 end

src/models/nodes.cr

@@ -1,5 +1,5 @@
 module Nodes
-  alias Leaf = Text | Image | IFrame | Anchor | UserAnchor
+  alias Leaf = Text | Image | IFrame
   alias Child = Container | Leaf | Empty
   alias Children = Array(Child)
 
@@ -136,15 +136,14 @@ module Nodes
     end
   end
 
-  class Anchor
+  class Anchor < Container
     getter href : String
-    getter text : String
 
-    def initialize(@href : String, @text : String)
+    def initialize(@children : Children, @href : String)
     end
 
     def ==(other : Anchor)
-      other.href == href && other.text == text
+      other.children == children && other.href == href
     end
 
     def empty?
@@ -152,18 +151,17 @@ module Nodes
     end
   end
 
-  class UserAnchor
+  class UserAnchor < Container
     USER_BASE_URL = "https://medium.com/u/"
 
     getter href : String
-    getter text : String
 
-    def initialize(userId : String, @text : String)
+    def initialize(@children : Children, userId : String)
       @href = USER_BASE_URL + userId
     end
 
     def ==(other : UserAnchor)
-      other.href == href && other.text == text
+      other.children == children && other.href == href
     end
 
     def empty?