migrate Node impl to alloy-primitives type and simplify

[ralexstokes]

Fixes #114.

Primarily, migrate Node to alloy_primitives::B256 to drop how much custom code we need

Also starts to lay the ground work for a similar refactor in ethereum-consensus to drop more custom serde code there (particularly around bytes and hex munging).

[codecov]

Codecov Report

Attention: Patch coverage is 77.96610% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 64.71%. Comparing base (13d2c7d) to head (644a576).

❗ Current head 644a576 differs from pull request most recent head 2c5d56c. Consider uploading reports for the commit 2c5d56c to get more accurate results

Files	Patch %	Lines
ssz-rs/src/merkleization/multiproofs.rs	0.00%	8 Missing ⚠️
ssz-rs/src/merkleization/merkleize.rs	66.66%	3 Missing ⚠️
ssz-rs/src/merkleization/node.rs	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   65.86%   64.71%   -1.16%     
==========================================
  Files          20       19       -1     
  Lines        1201     1176      -25     
==========================================
- Hits          791      761      -30     
- Misses        410      415       +5     

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