Allow to store references and eventually delete them

[make-github-pseudonymous-again]

Note that this has a significant cost on deletion operations, but not so much on insertion operations. Will have to investigate whether this cost can be reduced. This fixes #100.

PS: The slow-down comes from root fixing and pointer swapping instead of key-swapping.

[codecov]

Codecov Report

Merging #110 (cab9942) into main (a5c59c5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main      #110    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           33        36     +3     
  Lines         1351      1572   +221     
==========================================
+ Hits          1351      1572   +221     

Impacted Files	Coverage Δ
src/deletion/delete_case0.js	100.00% <100.00%> (ø)
src/deletion/delete_case1.js	100.00% <100.00%> (ø)
src/deletion/delete_case2.js	100.00% <100.00%> (ø)
src/deletion/delete_case3.js	100.00% <100.00%> (ø)
src/deletion/delete_case4.js	100.00% <100.00%> (ø)
src/deletion/delete_case5.js	100.00% <100.00%> (ø)
src/deletion/delete_no_child.js	100.00% <100.00%> (ø)
src/deletion/replace_node.js	100.00% <100.00%> (ø)
src/index.js	100.00% <100.00%> (ø)
src/insertion/insert_case0.js	100.00% <100.00%> (ø)
... and 13 more

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