Use arbitrary_len to avoid arbitrary VariableLists overconsuming input

[macladson]

The current arbitrary impl for VariableList often consumes large amounts of the available Unstructured buffer due to selecting a random usize then capping at the max size of the list. Certain types using VariableList can contain very large max sizes (such as MaxBytesPerTransaction) and so these lists tend to completely consume the available buffer immediately and then pad with zeroes.

We should change the implementation to use arbitrary_len which scales the size of the returned value with the size of the remaining Unstructured buffer. This ensures we end up with many smaller lists vs one giant list filled mostly with zeroes.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 50.00%. Comparing base (6b615bf) to head (737cb56).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/variable_list.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   49.89%   50.00%   +0.10%     
==========================================
  Files          13       13              
  Lines         463      462       -1     
==========================================
  Hits          231      231              
+ Misses        232      231       -1     

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