cmark: updated to 0.30.3

0.30.3

* Fix quadratic complexity bug with repeated `![[]()`.
  Resolves CVE-2023-22486. Add new pathological test.

* Allow declarations with no space, as per spec.

* Set `enumi*` counter correctly in LaTeX output.

* Allow `<!DOCTYPE` to be case-insensitive. (This conforms to the
  existing spec.)

* Fixed HTML comment scanning. Need to handle this case: `<!--> and -->`.
  Since the scanner finds the longest match, we had to
  move some of the logic outside of the scanner.

* Fix quadratic parsing issue with repeated `<!--` (this was not
  introduced by the previous fix, and not in a released version of cmark).
  Resolves CVE-2023-22484. Add new pathological test.

* Update HTML comment scanner to accord with commonmark/commonmark-spec#713.

* Pathological tests: half the number of repetitions, and the timeout.
  This reduces the time needed for the pathological tests.

* Shrink `struct cmark_node`. The `internal_offset` member is
  only used for headings and can be moved to `struct cmark_heading`.
  This reduces the size of `struct cmark_node` from 112 to 104 bytes on
  64-bit systems.

* Add `-Wstrict-prototypes` and fix offending functions.

* Fix quadratic behavior involving `get_containing_block`.
  Instead of searching for the containing block, update the tight list
  status when entering a child of a list item or exiting a list.

* Fix `pathological_tests.py`:
  - Use a multiprocessing.Queue to actually get results from spawned
    tests processes.
  - Fix the `allowed_failures` test.
  - Truncate actual output when printed.
  - Prepare for testing pathological behavior of the Commonmark renderer.

* Fix source position bug with backticks (kyle).

(adam)