Rename `into` methods to `onto`

[Nemo157]

To allow space for future impls of Into or an inherent equivalent

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (aeb711c) 72.91% compared to head (afac4b8) 72.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #109 +/- ## ======================================= Coverage 72.91% 72.91% ======================================= Files 4 4 Lines 288 288 ======================================= Hits 210 210 Misses 78 78 ``` | [Impacted Files](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157) | Coverage Δ | | |---|---|---| | [src/lib.rs](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-c3JjL2xpYi5ycw==) | `100.00% <ø> (ø)` | | | [src/decode.rs](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-c3JjL2RlY29kZS5ycw==) | `64.92% <100.00%> (ø)` | | | [src/encode.rs](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-c3JjL2VuY29kZS5ycw==) | `81.60% <100.00%> (ø)` | |

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