Add lineage tail

[jeromekelleher]

More work for #2294

[codecov]

Codecov Report

Attention: Patch coverage is 86.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.94%. Comparing base (a24cf0f) to head (07f7e59).

Files	Patch %	Lines
lib/msprime.c	86.00%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2298      +/-   ##
==========================================
- Coverage   90.96%   90.94%   -0.03%     
==========================================
  Files          20       20              
  Lines       12032    12043      +11     
  Branches     2449     2450       +1     
==========================================
+ Hits        10945    10952       +7     
- Misses        600      602       +2     
- Partials      487      489       +2     

Flag	Coverage Δ
C	90.94% <86.00%> (-0.03%)	⬇️
python	98.70% <ø> (ø)

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[jeromekelleher]

Merging this as a working midpoint. Still refactoring to abstract out the hull concept for SMC K and generally tidy things up.