feat: new package encoding (#78)

* feat: new package encoding

* refactor: using `@math` instead of misusing `@int`

* feat: support show for Chars and drop `try_collect`

* refactor: reduce encoding bytes allocation overhead

* feat: Malformed Bytes instead of String

due to unsafe substring

* refactor: use param name `bytes` instead of `buf`

* feat: new api `to_utf{8,16,16le,16be}_bytes`

`write_utf{8,16,16le,16be}_char`: reduce `write_bytes` allocation overhead

* refactor: reduce necessary decode

* minor: add some comments on Decoder

* fix: utf16be encoding

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Co-authored-by: Yorkin <[email protected]>