perf: avoid unnecessary clone when building Merkle trees

[Oppen]

The function complete_until_power_of_two takes a mutable reference, modifies the Vec<_> in-place, only to clone it and return it. Instead, just take ownership of the argument and return it again, without cloning.

Note while the change is marked as an optimization the impact isn't that high (<2%), but it's almost free and "feels" a little bit more correct this way.

Type of change

Please delete options that are not relevant.

• New feature
• Bug fix
• Optimization

Results

Ran on M1 MBP 2020 14" 16GiB:

Merkle Tree/build       time:   [724.84 ms 734.25 ms 748.73 ms]                                                        
                        change: [-3.1234% -1.9869% -0.6644%] (p = 0.01 < 0.05)                                         
                        Change within noise threshold.

Checklist

• Linked to Github Issue
• Unit tests added
• This change requires new documentation.
  • Documentation has been added/updated.
• This change is an Optimization
  • Benchmarks added/run

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.03%. Comparing base (7ffcf7c) to head (c147ffe).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #872   +/-   ##
=======================================
  Coverage   72.03%   72.03%           
=======================================
  Files         149      149           
  Lines       33407    33407           
=======================================
  Hits        24066    24066           
  Misses       9341     9341           

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