Pulls in upstream changes:
cmark 0.5.6.3 (13 Mar 2019)
- Use snoc instead of <> to add the null terminator.
cmark 0.5.6.2 (13 Mar 2019)
- Ensure that withtext adds a null terminator (jgm#14).
This fixes a regression due to PR jgm#13, which caused
random content to appear in rendered TEXT nodes.
