Remove methods that reimplement underlying functionality of List and Vector

[ralexstokes]

Originally, the mutable APIs into the underlying type that List and Vector wrap were specialized to the local wrapping types so that mutations could be tracked along the way to support caching a la #17.

I'd like to release a v1 of this library and won't have caching support so go ahead and drop the specialized implementations here, in lieu of just exposing the full functionality of the wrapped types.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 64.02%. Comparing base (fddda62) to head (364270b).

Files	Patch %	Lines
ssz-rs/src/list.rs	50.00%	1 Missing ⚠️
ssz-rs/src/vector.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
- Coverage   64.32%   64.02%   -0.31%     
==========================================
  Files          20       20              
  Lines        1183     1173      -10     
==========================================
- Hits          761      751      -10     
  Misses        422      422              

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