Add support for smallvec::SmallVec

[mina86]

Introduce smallvec cargo feature which pulls in smallvec crate and implements EncodeTarget and DecodeTarget for smallvec::SmallVec. With that, it’s now possible to use encoder’s and decoder’s into methods to write data into a smell vector. This may be used to prevent heap allocations for short data while properly handling larger data.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.48 :tada:

Comparison is base (8fbe049) 68.59% compared to head (533562a) 70.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #99 +/- ## ========================================== + Coverage 68.59% 70.07% +1.48% ========================================== Files 4 4 Lines 242 254 +12 ========================================== + Hits 166 178 +12 Misses 76 76 ``` | [Impacted Files](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/99?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157) | Coverage Δ | | |---|---|---| | [src/decode.rs](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/99?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-c3JjL2RlY29kZS5ycw==) | `60.68% <100.00%> (+2.12%)` | :arrow_up: | | [src/encode.rs](https://app.codecov.io/gh/Nullus157/bs58-rs/pull/99?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nullus157#diff-c3JjL2VuY29kZS5ycw==) | `79.62% <100.00%> (+1.19%)` | :arrow_up: |

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