Support Miri

[abonander]

Miri is currently choking on the use of is_x86_feature_detected!() here: https://github.com/seanmonstar/httparse/blob/master/src/simd/mod.rs#L74

Miri is probably not going to be supporting SIMD anytime soon anyway so it'd be nice if we could use #[cfg(miri)] to turn off feature detection entirely and just use the naive algorithms.

This could potentially happen automatically but this crate may need attention in other places in order to work in Miri.