Reduce use of "magic numbers"

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.22 :tada:

Comparison is base (7284f01) 73.90% compared to head (c05f979) 74.12%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #86 +/- ## ========================================== + Coverage 73.90% 74.12% +0.22% ========================================== Files 18 18 Lines 820 827 +7 ========================================== + Hits 606 613 +7 Misses 214 214 ``` | [Impacted Files](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?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/lib.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9saWIucnM=) | `100.00% <ø> (ø)` | | | [ssz-rs/src/merkleization/mod.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9tZXJrbGVpemF0aW9uL21vZC5ycw==) | `90.80% <ø> (ø)` | | | [ssz-rs/src/ser.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9zZXIucnM=) | `70.27% <ø> (ø)` | | | [ssz-rs/src/bitlist.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9iaXRsaXN0LnJz) | `79.16% <100.00%> (+0.44%)` | :arrow_up: | | [ssz-rs/src/bitvector.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9iaXR2ZWN0b3IucnM=) | `82.95% <100.00%> (+0.39%)` | :arrow_up: | | [ssz-rs/src/list.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9saXN0LnJz) | `77.10% <100.00%> (+0.27%)` | :arrow_up: | | [ssz-rs/src/uint.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy91aW50LnJz) | `88.31% <100.00%> (+0.31%)` | :arrow_up: |

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