Cache pandoc-citekey

[mdbenito]

As a followup to #238, this PR shaves off a few more seconds from the build by unobtrusively using functools.lru_cache to memoize the rendering of citekeys. For our 32 bib entries, and ~30 sec build this cuts time down by 8-10%.

The size of 1024 is of course arbitrary, but I assume more than enough for the majority of projects. Alternatively one can set it to None and let the cache grow indefinitely (which shouldn't be a problem in principle, but better safe than sorry :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.00%. Comparing base (1496fab) to head (a97e81f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   97.97%   98.00%   +0.02%     
==========================================
  Files           3        3              
  Lines         198      200       +2     
==========================================
+ Hits          194      196       +2     
  Misses          4        4              

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