Remove hullend struct

[jeromekelleher]

Initial work on #2383

As I work through it, I wonder if we can save some time on AVL tree maintenance by storing pointers to the left and right AVL tree nodes in the hull. It's not much space and would simplify things quite a bit.

Should probably work it out on the Python implementation first though.

[codecov]

Codecov Report

Attention: Patch coverage is 95.41284% with 5 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
lib/msprime.c	95.41%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2384   +/-   ##
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  Coverage        ?   91.22%           
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  Files           ?       20           
  Lines           ?    11834           
  Branches        ?     2294           
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  Hits            ?    10795           
  Misses          ?      570           
  Partials        ?      469           

Flag	Coverage Δ
C	91.22% <95.41%> (?)
python	98.68% <ø> (?)

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[jeromekelleher]

Closes #2383

[jeromekelleher]

This also embeds the AVL nodes needed by the hull in the left and right AVL trees, which significantly reduces the need for searching around and simplifies the code.