Delegate VariableList arbitrary impl to Vec<T>

[macladson]

While #81 did help constrain list lengths, testing in sigp/lighthouse#9006 revealed that arbitrary VariableList lengths were still far too large.

A better approach is to delegate to arbitrary's own Vec<T> implementation, which constrains list lengths even more by using an iterative coin flip strategy. This keeps list lengths small enough to not immediately consume the whole buffer when constructing blocks.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.10%. Comparing base (fc690e9) to head (230bd46).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/variable_list.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   50.00%   50.10%   +0.10%     
==========================================
  Files          13       13              
  Lines         462      461       -1     
==========================================
  Hits          231      231              
+ Misses        231      230       -1     

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