Add a leaf type for raw string content.

 - Add an abstract leaf type for leaves containing only raw
(unparsable) string content.
Rearrange code block types to match the spec.

 - Swap code block child structure from inline content to raw string
content.
 - Add an explicit type for fenced code blocks.
 - Rename language code block type to the more accurate name info code
block.
Make HTML blocks raw string content leaves.
Add missing inline content traits to various types

 - Make autolink, raw HTML, and unparsed inline content all implement
the inline content trait interface.
Allow unparsed inline content to append text.
Add tests for list item markers.
Add tests for code blocks.
Add tests for HTML blocks.
Add tests for all levels of header blocks.