perf(codec): improve performance by storing next index

[JohnTitor]

PR Type

perf

PR Checklist

Check your PR fulfills the following:

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt

Overview

Improve performance from O(n^2) to O(n) by storing next_index in LinesCodec. The idea is from https://docs.rs/tokio-util/latest/src/tokio_util/codec/lines_codec.rs.html.
Didn't touch the changelog as it's unreleased yet.

[robjtede]

thoughts on a .reset() method, in case someone legit wants to use the same struct for a new buffer ?

[JohnTitor]

After some thinking of this, it'd be a bit tricky to support such a usecase as it might cause index corruption. In d26ccb6, I tightened its availability for external use for now. Maybe we could revisit this topic if someone wants to use it.