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),
     ]