feat: derive Hash for merkle node

[jacobkaufmann]

while Node represents a hash itself, it would be useful for Node to implement Hash. for example, there are multiple instances in the ethereum consensus spec where we would like to key by the Root of some data structure.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -1.18 :warning:

Comparison is base (8aa7f50) 75.55% compared to head (8487016) 74.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## ========================================== - Coverage 75.55% 74.38% -1.18% ========================================== Files 18 18 Lines 855 808 -47 ========================================== - Hits 646 601 -45 + Misses 209 207 -2 ``` | [Impacted Files](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes) | Coverage Δ | | |---|---|---| | [ssz-rs/src/merkleization/node.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9tZXJrbGVpemF0aW9uL25vZGUucnM=) | `60.00% <ø> (ø)` | | ... and [8 files with indirect coverage changes](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/90/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes)

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