Implement improved mrca.

[jeromekelleher]

Closes #1309

Implements @feilchenfeldt's beautifully elegant mrca algorithm discussed in #1306

Would you mind taking a look at the diff please @feilchenfeldt?

[codecov]

Codecov Report

Merging #1313 (4a0a5de) into main (759f0bb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1313   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files          26       26           
  Lines       22189    22174   -15     
  Branches     1006     1006           
=======================================
- Hits        20816    20803   -13     
+ Misses       1340     1338    -2     
  Partials       33       33           

Flag	Coverage Δ
c-tests	92.44% <100.00%> (+<0.01%)	⬆️
lwt-tests	92.97% <ø> (ø)
python-c-tests	95.14% <ø> (ø)
python-tests	98.86% <ø> (ø)

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Impacted Files	Coverage Δ
c/tskit/trees.c	94.87% <100.00%> (+0.04%)	⬆️

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[feilchenfeldt]

The algorithm looks good to me.

[benjeffery]

LGTM, great stuff.
PInging @JSaha20 as it was mentioned she would be interested in the changes.

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