Revert "Remove distinction btw atx and setext header in AST."

This reverts commit a71423f6ee1b77d9f79d42599ea00b4ca99f5da0.

Not quite sure about this change, so reverting for now.
Note that we still have a distinction between fenced and
indented code blocks in the AST.  These two distinctions
seem to stand or fall together.