Adds print elision logic for similar consecutive nodes.

[ghyatzo]

Hello, this PR aims at adding the ability to elide printing siblings that are similar.

at the moment it introduces the following behaviour:

• after maxsiblings occurrences of similar node values start eliding

• if a different enough node value appears reset the counter and start printing again

• elided siblings are not recursed into.

• users can define a nodevalue_distance function similar to how they can override printnode to define what "similar" means. it defaults to equality. Users can also pass in a threshold to finetune when things are going to be elided.

fixes: #144

initial prototype built with a robot friend, finetuned by hand.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.30%. Comparing base (5ed4c69) to head (f06f3f2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
+ Coverage   70.69%   72.30%   +1.61%     
==========================================
  Files           8        8              
  Lines         430      455      +25     
==========================================
+ Hits          304      329      +25     
  Misses        126      126              

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