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

This reverts commit 4570eb2bff2e1b71fa5b6408abbc69c98ff5ff24.
Do not distinguish btw fenced and indented code in AST.

Use a single CMARK_NODE_CODE_BLOCK tag for both.
Distinguish them when needed for parsing by looking at the
fence_length attribute, which is 0 for indented blocks.