Httparse

- **FEAT**: Adds `ParserConfig`, a new type to allow making some parsing rules more or less strict, or otherwise customize the parsing.
- **FEAT**: Adds `ParserConfig::allow_spaces_after_header_name_in_responses(bool)`, which when set to `true`, will allow skipping spaces between a header name and the colon. The specification requires that to be illegal in most cases, but says proxies should accept and remove those spaces.

- **FIX**: Set `Response.reason` to an empty string if `obs-text` is found in the reason-phrase.
- **PERF**: Fix faster `next_8` to work when there are exactly 8 bytes left.

- **FIX**: Allow backslash (`\`) characters when parsing request-target.

- **FIX**: Allow {|}^` characters when parsing request-target.
- **FIX**: Strip trailing whitespace from header values.
- **PERF**: Improve selection of AVX2 and SSE4.2 features if both are available.

- **FIX** fix incorrect response parsing when reason-phrase is missing.

- **FIX** fix compile error of AVX2 functions on 32bit targets.