Support tree hash by value

[michaelsproul]

Companion to:

• Change trait to take self by value tree_hash#23

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.16%. Comparing base (7758a1e) to head (4ad1965).
⚠️ Report is 40 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bitfield.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   61.08%   61.16%   +0.07%     
==========================================
  Files           9        9              
  Lines         496      497       +1     
==========================================
+ Hits          303      304       +1     
  Misses        193      193              

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