Rename tsk_tree_t.left/right to interval.left

[jeromekelleher]

Closes #1686

Doesn't look too bad noise-wise, and seems worth the update?

I wondered if there was something fancy we could do with unions that would allow us to use either tree.interval.left or tree.interval[0]``, depending on the need, but I don't think it's possible in C99 (and would be of very marginal benefit anyway)

[codecov]

Codecov Report

Merging #1913 (5858323) into main (6a803c6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1913      +/-   ##
==========================================
- Coverage   93.16%   93.16%   -0.01%     
==========================================
  Files          27       27              
  Lines       25094    25093       -1     
  Branches     1109     1109              
==========================================
- Hits        23379    23378       -1     
  Misses       1681     1681              
  Partials       34       34              

Flag	Coverage Δ
c-tests	92.21% <100.00%> (-0.01%)	⬇️
lwt-tests	89.14% <ø> (ø)
python-c-tests	68.29% <100.00%> (ø)
python-tests	98.78% <ø> (ø)

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Impacted Files	Coverage Δ
c/tskit/haplotype_matching.c	95.07% <100.00%> (ø)
c/tskit/trees.c	94.18% <100.00%> (-0.01%)	⬇️
python/_tskitmodule.c	90.38% <100.00%> (ø)

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[benjeffery]

LGTM!