Add TreeHashMut

[michaelsproul]

Define a new TreeHashMut trait which takes &mut self

This might be a cleaner alternative to:

• Change trait to take self by value #23

Although we got bogged down in the Milhouse changes, especially around supporting nested lists: List<List<u64, N>, M> is troublesome because the T in List<T> should probably implement TreeHashMut, but we don't have mutable access to the elements of an immutable list.

Another alternative might be to embrace interior mutability in Milhouse so that TreeHash can be implemented without mutation altogether.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.14%. Comparing base (12d828c) to head (6ea7d0f).

Files with missing lines	Patch %	Lines
tree_hash/src/lib.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   79.66%   78.14%   -1.52%     
==========================================
  Files           6        6              
  Lines         295      302       +7     
==========================================
+ Hits          235      236       +1     
- Misses         60       66       +6     

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