Remove strong reference cycle

[humdrum]

What this PR does / why we need it:

• Remove memory leaks caused by strong reference cycles of Trees.
• Set parents as weak to prevent a strong reference cycle.
• Set prev and next of RGATreeSplit too.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (38ea3c2) 44.24% compared to head (3318bda) 44.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #118   +/-   ##
=======================================
  Coverage   44.24%   44.24%           
=======================================
  Files         105      105           
  Lines       22229    22229           
=======================================
  Hits         9836     9836           
  Misses      12393    12393           

Files Changed	Coverage Δ
Sources/Document/CRDT/CRDTTree.swift	66.25% <ø> (ø)
Sources/Document/CRDT/RGATreeSplit.swift	77.66% <ø> (ø)
Sources/Util/LLRBTree.swift	86.68% <ø> (ø)
Sources/Util/SplayTree.swift	87.05% <ø> (ø)
Sources/Util/Trie.swift	85.52% <ø> (ø)

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