Speedup headers parsing

Speedup headers parsing

Replace `_CRLF_RE.split(headers)` with a simple `headers.split('\n')` with an additional check for `'\r'` in each line.
Add benchmark to measure performance impact of the change.
The benchmark results are as follows:
* split only: ~3x faster on CPython, 14-18x faster on PyPy
* full headers parse: ~1.3x faster on CPython, ~3-4.5x faster on PyPy