Change LineDecoder to match stdlib splitlines, resulting in significant speed up...

Change LineDecoder to match stdlib splitlines, resulting in significant speed up (#2423)

* Replace quadratic algo in LineDecoder

Leading to enormous speedups when doing things such as
Response(...).iter_lines() as described on issue #2422

* Update httpx/_decoders.py

* Update _decoders.py

Handle text ending in `\r` more gracefully.
Return as much content as possible.

* Update test_decoders.py

* Update _decoders.py

* Update _decoders.py

* Update _decoders.py

* Update httpx/_decoders.py

Co-authored-by: cdeler <serj.krotov@gmail.com>
* Update _decoders.py

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Co-authored-by: Tom Christie <tom@tomchristie.com>
Co-authored-by: cdeler <serj.krotov@gmail.com>