Introduce an overall limit to link. ref. defs instantiations.

[mity]

This is to prevent time and output size explosion in case of input pattern generated by this:

$ python -c 'N=1000; print("[x]: " + "x" * N + "\n[x]" * N)'

We roughly allow to blowing up the input size of the document 16 times by link reference definitions or up to 1 MB, whatever is smaller. When the threshold is reached, following reference definitions are sent to output unresolved as a text.

Fixes #238.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6c29f60) 91.76% compared to head (b71ceef) 91.79%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
+ Coverage   91.76%   91.79%   +0.02%     
==========================================
  Files           5        5              
  Lines        3402     3412      +10     
==========================================
+ Hits         3122     3132      +10     
  Misses        280      280              

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