feat(codec): use `IntoBig`

[itsdevbear]

Closes: https://github.com/karalabe/ssz/issues/7

[codecov[bot]]

Codecov Report

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

Project coverage is 83.85%. Comparing base (026e397) to head (6faa5b6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #17 +/- ## ========================================== - Coverage 83.92% 83.85% -0.07% ========================================== Files 61 61 Lines 3035 3035 ========================================== - Hits 2547 2545 -2 - Misses 323 324 +1 - Partials 165 166 +1 ``` | [Components](https://app.codecov.io/gh/karalabe/ssz/pull/17/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | Coverage Δ | | |---|---|---| | [ssz](https://app.codecov.io/gh/karalabe/ssz/pull/17/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | `83.11% <100.00%> (-0.13%)` | :arrow_down: | | [sszgen](https://app.codecov.io/gh/karalabe/ssz/pull/17/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | `70.99% <ø> (ø)` | |