refactor!(core): unify edge splits method error policy

[imrn99]

All variants of the edge-splitting op now return a boolean:

• true if the edge was correctly split
• false if it could not be split, and is left unchanged

Note that:

• the splitn_edge did not need to return a vector of dart identifiers, we can find those after the op by using beta 1
• the returned boolean gives an actual information to the user about the completion of the op. the user can then choose to retry or abort as needed

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.57143% with 21 lines in your changes missing coverage. Please review.

Project coverage is 81.54%. Comparing base (01ece1f) to head (9d66b08).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
honeycomb-core/src/cmap/dim2/advanced_ops.rs	57.14%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
- Coverage   81.71%   81.54%   -0.18%     
==========================================
  Files          36       36              
  Lines        5338     5293      -45     
==========================================
- Hits         4362     4316      -46     
- Misses        976      977       +1     

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