Refactor mark chains.

[mity]

• Rename MD_MARKCHAIN to MD_MARKSTACK to indicate its semantics much clearer.
• Simplify its implementation (single-linked list instead of double-linked one).
• Where it was reused (misused?) for other, unrelated stuff, with other semantics, it's now done explicitly. (i.e. got rid of TABLECELLBOUNDARIES).
• PTR_CHAIN still uses the stack (we don't care about order there), but it got separated from the array of ordinary opener stacks at least.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (23b1416) 94.74% compared to head (f34c0cc) 94.77%. Report is 2 commits behind head on master.

Files	Patch %	Lines
src/md4c.c	99.21%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #224 +/- ## ========================================== + Coverage 94.74% 94.77% +0.03% ========================================== Files 3 3 Lines 3141 3141 ========================================== + Hits 2976 2977 +1 + Misses 165 164 -1 ```

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