fix?: use bt structure in BEEF impl

[arkadiuszos4chain]

Replace bux structs with bt.

[codecov]

Codecov Report

Merging #435 (13f64a8) into tests-DO-NOT-MERGE-250-beef (52f3a4b) will decrease coverage by 0.05%.
The diff coverage is 77.14%.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           tests-DO-NOT-MERGE-250-beef     #435      +/-   ##
===============================================================
- Coverage                        52.69%   52.64%   -0.05%     
===============================================================
  Files                               96       96              
  Lines                            10716    10718       +2     
===============================================================
- Hits                              5647     5643       -4     
- Misses                            4629     4635       +6     
  Partials                           440      440              

Flag	Coverage Δ
unittests	52.64% <77.14%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
beef_tx_sorting.go	100.00% <100.00%> (ø)
beef_tx_bytes.go	78.33% <83.33%> (+3.73%)	⬆️
beef_tx.go	55.26% <53.33%> (-9.53%)	⬇️

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