allow optional `0x` prefix on hex {en,de}coding

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9687857) 75.67% compared to head (4572f39) 75.76%.

:exclamation: Current head 4572f39 differs from pull request most recent head c0a21cb. Consider uploading reports for the commit c0a21cb to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #115 +/- ## ========================================== + Coverage 75.67% 75.76% +0.08% ========================================== Files 17 17 Lines 847 846 -1 ========================================== Hits 641 641 + Misses 206 205 -1 ``` | [Files](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/115?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/serde.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/115?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9zZXJkZS5ycw==) | `52.38% <100.00%> (+0.20%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/115/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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