Eliminate bucket relay of window

[femshima]

TODO: remove thisとなっている部分は，今後LabelとSStreamSetのリファクタリングをする際に消す予定です．

参考までにベンチマークの結果を．

main

test bonsai        ... bench:  17,225,324 ns/iter (+/- 71,784)
test bonsai_letter ... bench:  47,869,986 ns/iter (+/- 102,811)
test is_bonsai     ... bench:  26,584,675 ns/iter (+/- 73,453)

4b8b1e3

test bonsai        ... bench:  16,898,101 ns/iter (+/- 35,576)
test bonsai_letter ... bench:  47,021,735 ns/iter (+/- 240,455)
test is_bonsai     ... bench:  26,094,654 ns/iter (+/- 92,799)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.85%. Comparing base (fa9b6ec) to head (70d95d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   68.78%   68.85%   +0.06%     
==========================================
  Files          26       27       +1     
  Lines        1496     1496              
==========================================
+ Hits         1029     1030       +1     
+ Misses        467      466       -1     

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